Wikipedia:Reference desk/Archives/Science/2013 December 10

Welcome to the Wikipedia Science Reference Desk Archives
The page you are currently viewing is an archive page. While you can leave answers for any questions shown below, please ask new questions on one of the current reference desk pages.

December 10

% new cases of HIV resulting from MSM globally

I found this data for the US: More than 50% of all new HIV infections occur in gay men and other men who have sex with men (MSM). This is from amFAR.

And this for the Philippines, Eighty-five percent of the new cases of HIV in the Philippines this year involve MSM.

Would like to know data on the global figures.

Thank you. — Preceding unsigned comment added by 58.69.95.234 (talk) 02:15, 10 December 2013 (UTC)

Hello! Your statement here has confused us. Are you asking us to tell you about HIV? Are you offering to provide us with information. If you have information to give us, please visit talk:HIV and offer the information there. Thank you. μηδείς (talk) 02:24, 10 December 2013 (UTC)

Sorry I removed the "you". Would like to know data on the global figures it should have said.

This publication by the UN has some statistics. I doubt you're going to find truly global numbers published anywhere, but this report has some numbers and charts for different parts of the world starting on page 22.

Hope this helps. APL (talk) 02:39, 10 December 2013 (UTC)

Thanks APL. I saw that before, but didn't find any data on this aspect. — Preceding unsigned comment added by 58.69.95.234 (talk) 03:05, 10 December 2013 (UTC)

maximum tree height

Please tell me: when I fly over a forest, trees seem to "agree" that none of them is going to tower over all the others. Would it not be advantageous for one of them to grow just a bit taller than the others in able to capture more sunlight? — Preceding unsigned comment added by 67.86.202.189 (talk) 14:53, 10 December 2013 (UTC)

Trees are not all the same height, but a given species can grow to a range of heights. Natural (fire) and artificial (lumbering) clear-cutting tends to restart forested areas at around the same time, so it's not surprising that a given area will tend to have trees of similar height. ←Baseball Bugs ^{What's up, Doc?} carrots→ 15:21, 10 December 2013 (UTC)

Sure, but there's more to it than that. There is a very strong competition for light, but biological factors (wood strength, growth rate, ability to transport water upward, etc.) limit how high any given species can get. So when you have a single dominant species in an old forest, you tend to get uniformity. But if there is a mix of species, you often see some trees rising above the canopy. Looie496 (talk) 15:34, 10 December 2013 (UTC)

A few such trees would likely have little effect on the population, but a lot of them could crowd the shorter trees out in the long run, yes? Kind of like "volunteer corn" in a soybean field. A few stalks here and there won't have any appreciable overall effect. But planting corn and beans together could impare the growth of the bean plants. ←Baseball Bugs ^{What's up, Doc?} carrots→ 15:43, 10 December 2013 (UTC)

Some trees are adapted to live under the canopy and don't try to compete with the big trees. In European woodland, we talk about the "ground layer" which has plants like bramble and bracken and tree seedlings, followed by the "shrub layer" or "mid-layer" with holly, hazel, hawthorn and saplings of the big trees that are on the way up. Finally, there's the "canopy layer" which has the crowns of the big forest trees like oak and beech.[1] The king of the shrub layer trees is holly, which does most of its photosynthesis during the winter and early spring before the other trees get their new leaves. Alansplodge (talk) 16:05, 10 December 2013 (UTC)

The article you want is probably ecological succession and related linked articles. Matt Deres (talk) 17:48, 10 December 2013 (UTC)

• This is an open question, one that interested me as an undergrad and still fascinates me. One enigma is the dwarf pine section of the Pine Barrens (New Jersey). It's not known for sure if genetics or environment or both accounts for the fact that these pines only grow to about 6 feet while they grow to 30 or more in other places. μηδείς (talk) 19:09, 10 December 2013 (UTC)

• What you will find is there is a natural selection answer for everything, post-discovery. Very little is known a priori about what nature will select, only that what we observe has been selected. In addition, humans manipulate selections (naturally, not just advanced DNA manipulation) such as domesticated animals (broilers, dog breeds, cattle, horses, salmon, sheep, etc) and crops such as wheat, rice, oranges and corn and a lot of those selections are supported and controlled to an extent. What is harder is to take a manipulated animal or plant and predict which would survive naturally. Moreover there are animal species such as fish that don't stop growing (similar to your trees question and observation) yet we still observe limits and ranges in size for certain species. It's like the weather - we know after the storm, what drove it's entire history. Yet we lack the the ability to forecast beyond a few days. --DHeyward (talk) 14:57, 11 December 2013 (UTC)

Density of wood shop waste

Hello.

I need to design a cyclone separator to remove saw dust and wood shavings from a wood working shop. This can be designed wit applications such as http://aerosol.ees.ufl.edu/cyclone/section08.html or www.enq.ufsc.br/disci/eqa5313/cyclone.xls‎

What will the gas density or particle loading and particle size distribution be?

Thank you.Crossgrain (talk) 15:56, 10 December 2013 (UTC)

Since woodshop dust extraction systems are operated with the mass of wood dust much lower than the mass of air, as far as gas density is concerned, you can simply use the density of air.

Particle size distribution is another matter. You have not supplied sufficient information. You need to settle a number of questions that dramatically affect particle mean size and size distribution. Is the dust extraction for the general room space, or is it to be hose-connected to the dust outlets on each individual machine? Room space dust is much finer. What sort of machines? Dust generated by orbital sanders is very fine, like the flour you make cakes from. Dust generated by cross-cut sawing is typicaly 100 times more coarse. Not all shops will do both. Why do you need to design your own? For a commerical shop, or your own hobby shop? A shop with only one worker doing fine furniture work is very different to a commercial shop milling construction timberwork. I have made fine furniture as a hobby for 50 years. I have never had dust extraction - I simply don't need it with the mostly hand tool methods I use (though even hand tools generate airborn dust), and the timber I use, mostly the Australian timber Jarrah and radiata pine, are not-toxic. Woods used in some countries are toxic. In any case, operating a commercial shop with two or more people would in most countries invite prosecution for OH&S violations.

The American hobby magazine Fine Woodworking does reviews of commercial dust extraction at frequent intervals, and also have articles on selecting and installing dust extraction equipment. Their reviews are good because they cover testing methods and the particle size range expected in various situations. I suggest you search back issues over the last 2 years or so. Copies should be in the better public libraries.

I suggest you compare any calculated design you originate with typical commercial equipment to see if you are in the ballpark.

120.145.174.117 (talk) 23:14, 10 December 2013 (UTC)

Outer planets: conjunction

I have a couple of questions about the outer planets. I found this link here in Wikipedia on the Jupiter-Saturn conjunction page: http://geogebratube.org/student/m21235 which is a fantastic link but: 1) Is there any link in internet with a similar diagram showing conjunction positions of Uranus and Neptune as well? (with years if possible). This is a calculation I tried to make, is this statement correct?: 2) There will be a conjunction of Jupiter-Saturn-Uranus in 2260/2261, all three will then be closest to each other in a straight line together? 3) In that case above, it would be: Jupiter -> Saturn 4.5 au, Saturn -> Uranus 9.5 au (and therefore Earth -> Uranus: total 18 au)? ~~SGAst~~ — Preceding unsigned comment added by SGAst (talk • contribs) 16:27, 10 December 2013 (UTC)

See JPL Horizons On-Line Ephemeris System. This system can certainly calculate the position of the outer planets for any date you might be interested in. I don't know if it will automatically find conjunctions, or if you would have to calculate a series of positions perhaps at 1 year intervals, and then calculate at closer intervals for the years that look interesting. Jc3s5h (talk) 15:14, 13 December 2013 (UTC)

Exotic yeast gut flora

I've been reading about auto-brewery syndrome and was wondering whether any other exotic yeasts have been found in gut flora, or theoretically could. I was wondering specifically whether a stomach could, however rare and unlikely; create bread, provided the right ingredients. CensoredScribe (talk) 17:20, 10 December 2013 (UTC)

It's not particularly surprising that alcohol is produced in the gut, since alcoholic fermentation is such a simple process, the components for which are available all the time (alcohol is almost certainly being produced in your gut as you read this). In contrast, bread production requires much more specific ingredients and conditions (including a relatively dry spot in which to bake). At the risk of arguing from ignorance, it seems extremely unlikely that intestines would produce anything we would recognize as bread. -- Scray (talk) 19:33, 10 December 2013 (UTC)

Especially since getting one's gut up to 425 degrees F is contraindicated by most medical professionals. --Jayron32 01:56, 11 December 2013 (UTC)

I suppose Spontaneous human combustion could be more likely in someone with auto brewery syndrome, though despite the name; none of the alleged cases have ever described a fire actually starting within the body, nor has there been a case where someone has survived such an incident. I guess there's a reason Mulder never tried to provide even the flimsiest of scientific explanations for the miracles in skeptical Sculley's religion. I mean other than the obvious answer; that such an explanation applied to Jesus would be extremely offensive to the predominately conservative audience of Fox. CensoredScribe (talk) 22:34, 11 December 2013 (UTC)

"Discovering" the obvious

This may sound like a non-ref-desk-friendly question, but I'm looking for third-party sources on the topic, not speculation.

Basically what I'm wondering is what sources have to say about how totally obvious things can remain undiscovered (or at least not widely understood) for such a long period of time. I'll give two examples:

When I was a kid, I started drawing my own bath at a pretty young age. I quickly learned that I couldn't fill the tub up to the rim, because once I got in the tub, the mass of my body would dislocate much of the water, causing the tub to overflow. This was more than a decade before I'd heard of some old dead guy named Archimedes. My "discovery" didn't take any specificially modern comprehension of how water and solid matter interact, an understanding of the principle of buoyancy, or anything of the sort. I simply observed a reaction (the tub overflowing) to something I had done (failing to account for my mass in filling the tub) and reached a simple conclusion: water takes up space as much as people do, and since two things can't be in the same place at the same time, the water is going to have to get out of the way.

Had I been alive in ancient Greece, and had I thought to mention my "discovery" to someone who realized its relevance to science, that would be my principle, not Archimedes'. Never mind that I was six when I discovered it!

The second example has to do with the anatomical blind spot. I've always been almost blind in my right eye. Put a patch over my left eye and place a book more than six inches away from my face, and I'm completely helpless trying to read what's on the page. This led to a number of interesting quirks. If I'm squinting in the sunlight, for example, I'm likely to close my right eye, regardless of which side of me the sun is on, so the visual accuity I have with my left eye remains. I also handle a bow and arrow like a left-hander, even though I'm right-handed, since I need to have my one good eye most closely aligned with the trajectory of the arrow.

Around the time I was ten, I discovered that when I closed my right eye, I could move my left eye around in such a way as to make objects in a certain position disappear. What I was observing was the effect of the anatomical blind spot, the point at which the optic nerve leaves the eye, devoid of receptor cells. This is a phenomenon that somehow managed to remain unknown to scientists until 1663, when it was described by Edme Mariotte.

Now, granted, I knew nothing of optic nerves or rods or cones in the second instance, but how is it that such totally obvious things remain unnoticed by science for so long?198.86.53.67 (talk) 18:30, 10 December 2013 (UTC)

the Koch curve

• Noticing things, baths overflow, apples fall, is different from formulating them mathematically and explaining them scientifically and publishing them. People can have intuitive grasps of things without being able to describe them explicitly. Also, there's priority. I discovered the Koch curve when I was 12. I only discovered it had been named and published--by someone else-- when I was 24. μηδείς (talk) 18:58, 10 December 2013 (UTC)

When they discovered that mummified hadrosaur a few years ago, and were so surprised by the shape of the animal, I was surprised that people were surprised: It had seemed perfectly obvious to me since I'd reconstructed hadrosaurs that way when I was about 12. — kwami (talk) 19:07, 10 December 2013 (UTC)

That's interesting. Here's a link to images of the hadrosaur. μηδείς (talk) 20:47, 10 December 2013 (UTC)

Displacement of water was known long before Archimedes. Aesop even has a fable of a crow using the principle, and crows are actually able to figure it out. What Archimedes realized was that this provided a way to distinguish between a complex shape of pure gold, and of gold adulterated with lead. That's not so obvious. — kwami (talk) 19:02, 10 December 2013 (UTC)

Science is not just noticing interesting phenomena but also methodically investigating them. The link describes how we conceive scientific method today but we also have articles History of scientific method and Timeline of the history of scientific method. Archimedes not only noticed his bath overflowing (the story goes) but also made an investigation about 250 BC to confirm his hypothesis which he stated as a general physical law, not limited to any particular bath: "Any object, wholly or partially immersed in a fluid, is buoyed up by a force equal to the weight of the fluid displaced by the object." Thus the scientist moves methodically from observation, to hypothesis, to testing the hypothesis, to arriving at a demonstrable explanation which has wider usefulness. This way of thinking is relatively recent in human history and a turning point is arguably the treatise Discourse on the Method by Descartes in 1637. Incidentally here is the correct article link about the eye's Optic disc (the OP has linked to "Optical disc" which is about CDs and DVDs.).84.209.89.214 (talk) 19:14, 10 December 2013 (UTC)

Some of the obvious things that people seemed to miss until recently include:

1) Plate tectonics, that is, that all the continents fit together like a jigsaw puzzle.

2) Ballistics, that a fired object follows a parabolic path. You would think they could tell this just by watching from the side as a trebuchet fired a stone, but they seemed to think the object went straight, or maybe followed a circular arc, until relatively recently.

3) Fractals (as hinted to above with the Koch Snowflake). The fractal nature of nature is quite obvious, once you know to look for it.

4) The the Earth is spherical. Anyone who has watched a ship sail away would have noticed it drop below the horizon. This only makes sense if the water is higher between us and the ship. In this case, this seems so counter-intuitive, that you can understand why people didn't want to accept it. StuRat (talk) 14:50, 11 December 2013 (UTC)

(4): I think people knew that the earth was round for much longer than modern people suspect. (3): The mathematics of Fractals were well understood for a long time - but it was impossible to comprehend how this math was important in the real world until we had fast enough computers (and especially computer graphics) to display them. When you see the Mandelbrot set and are able to zoom into it and explore it interactively, you can see things that look uncannily like fern leaves, coastlines, clouds and snowflakes and the connection to those real-world objects becomes such a visually compelling one that you're forced to ask "Why?" and discover the mechanisms of nature that result in the fractal nature of so many things. In many cases, it's the lack of tools (physical, mathematical or computational) to view these experiments in different ways that allows you to see what's "really" going on.

In the case of (1), Plate tectonics, we knew that some parts of the earth were moving relative to others because we could see earthquakes and folded mountain ridges. But only when you can see the direction that each entire plate is moving can you "put it together" and grasp the idea that these large, fairly rigid objects are obviously floating around on an underground sea of liquid. Without the data showing how small areas of the earth are moving relative to each other, all you see are little snapshots of local activity and understanding "the big picture" is extremely difficult.

In the case of ballistics (2), people clearly knew how to aim an arrow up into the air to hit a distant target - that's been known almost since the bow and arrow were first invented. Even chimpanzees and very small children with no knowledge of math, science and ballistics can toss a ball back and forth - with a precision that implies that at some level their brains and muscles understand that projectiles follow a parabolic path. But it required someone with the desire to describe it mathematically to discover what was really going on. The Greeks had the math skills and the ability to get a projectile on target - but their philosophical nature prevented them from applying one to the study of the other - so they never did "understand" what they knew full well how to do with their hands and actively used to conquer large parts of their world.

There are many other examples of this. The "chaotic" nature of the weather was only discovered when someone tried to simulate the progression of hurricanes on a computer - and decided to re-run a previous simulation by typing in the initial data a second time. Because he only typed in six significant digits instead of eight he got a WILDLY different result from his simulation - and realized that one part-per-million difference in the initial conditions could result in a 1000% difference in the results!

It's very often the case that we have everything we need to make some discovery - but it takes a small accident or some such thing to provoke a particular line of thinking that results in that discovery taking place.

A good example of this is Zipf's law - a simple mathematical relationship that seems to pop up in a lot of unexpected places (everything from the frequency at which common words are used in almost all human languages to the population sizes of our cities). We don't really know why that is...in a sense it's like chaos theory or fractals once were. We see some mathematical distribution almost everywhere we look, but we don't know why. Doubtless there are people excitedly trying to figure it out as we speak - but the catalyst for doing that wasn't some major new breakthrough, new data, new tools - it was just the happy coincidence of someone looking at some data and (as if by magic) seeing a simple relationship going on that looks as though it might explain something that's interesting.

That's not always the case - early astronomers thought they'd found a simple relationship between the sizes of the orbits of the planets and the diameters of platonic solids with the same length sides...but that turns out to be nothing more than a coincidence (and not an accurate one at that!)...but whenever something like that pops up, it's always worth looking at to see if there is something new to understand.

SteveBaker (talk) 15:37, 11 December 2013 (UTC)

We actually have an article on the spherical Earth, and the history of observations on this concept. To some extent, it's more a question of whose account was the first to be committed to permanent storage and not lost, rather than a matter of it really being discovered; the ancient Greeks definitely had it down by several lines of evidence by at least the 3rd century BC.

Worth noting is the frustrating tendency on the part of U.S. children's cartoons to suggest that Christopher Columbus' accidental discovery of 'America' was a consequence of a 'novel and controversial' belief by Columbus that the Earth was round, and that a route to the East Indies could be achieved by travelling west. In fact, the roundness of the Earth was well known and universally accepted by his fifteenth-century peers. Columbus' real new idea was based on – totally mistaken – calculations of the circumference of the Earth, which allowed him to suggest that the Earth was much smaller, and that the westward voyage would therefore be much shorter than his contemporaries believed. Columbus would have been an abject failure if the Americas hadn't interrupted his long westward sail; he was a lucky idiot. TenOfAllTrades(talk) 15:59, 11 December 2013 (UTC)

Fractals joke:

Q: What does the middle B stand for in "Benoît B. Mandelbrot"?

A: Benoît B. Mandelbrot.

HiLo48 (talk) 00:50, 13 December 2013 (UTC)

Good one! μηδείς (talk) 03:56, 13 December 2013 (UTC)

Citalopram limit

Antidepressant medication Citalopram has a warning by FDA not to exceed 40 mg daily. However, I recall reading an article saying that this limitation is overblown and is not warranted. Has anybody followed this development? I will appreciate any references? Thanks, AboutFace_22AboutFace 22 (talk) 19:46, 10 December 2013 (UTC)

Wikipedia (and people here at the reference desk) are required to follow "Reliable Sources" - and in this case, the FDA is amongst the top handful of most authoritative sources on the planet. So we pretty much have to go with what they say. I'm not sure where you saw these other numbers - certainly forums like StudentDoctor.net have much to say on the subject of the "STAR-D" controversy. I'm deeply concerned that people like Kevin Trudeau (a scientologist...not a scientist!) have been saying that the doses of drugs like this could be higher...which makes me even more inclined to believe the FDA! We cannot possibly endorse sources as flakey as that one! So unless someone here can come up with sources at least as reliable as the FDA, it would be irresponsible of us to say anything other than: "Listen to the FDA and ignore the crazy nut-jobs!" SteveBaker (talk) 21:32, 10 December 2013 (UTC)

Steve, as always you comment on a wide range of subjects! I believe, however that there is a flaw in your reasoning. I noticed the same problem in the pharmacist I am dealing with. FDA does not do research. What they did was: they collected literature, analyzed it and came up with this sweeping recommendation. I saw the article in question on Medscape though. It is sort of imperative for me to find it now but so far I haven't been successful. As far as I know the scientologists are opposed to all medications. Thanks, AboutFace_22AboutFace 22 (talk) 02:17, 11 December 2013 (UTC)

I absolutely agree that the FDA doesn't generally do research of their own. But that's not the point here. From a Wikipedia perspective...and especially on medicine-related topics (and double-especially here on the Reference Desks where there are additional restrictions in place), we prefer the kinds of sources mentioned in WP:MEDRS - which is to say "third-party" reviews of researched subjects. Specifically, our WP:MEDRS guidelines say: "Ideal sources for such content includes literature reviews or systematic reviews published in reputable medical journals, academic and professional books written by experts in the relevant field and from a respected publisher, and medical guidelines or position statements from nationally or internationally recognised expert bodies."...and clearly the FDA are amongst the latter. I'm not saying that the FDA are necessarily right - only that our answers here should be driven by their guidelines and those that are likely to come from similar 3rd party reviews of the material that's out there. Reports from single research studies are not the kinds of thing we should be discussing here. We need papers that summarize and review large numbers of studies - and organizations that report the results of such reviews...in short, the FDA. If we were to cite some single study that said that there were benefits from (say) twice the FDA recommended dose of some substance - then it's very possible that someone would follow that advice and kill themselves because of an error in the paper or a statistical mistake in it's analysis. That's why Wikipedia has very strict guidelines on that kind of thing, and these reference desks even more so. Which means that you're not going to get the answer you seek here unless some idiot comes along and violates our rules. SteveBaker (talk) 14:45, 11 December 2013 (UTC)

People vary tremendously in their responses to SSRIs, and usually the amount of data on toxicity for any individual member of the group is limited, so the FDA tends to err on the conservative side. Lots of people can't tolerate even the clinically recommended doses. Looie496 (talk) 02:34, 11 December 2013 (UTC)

Indeed. SteveBaker (talk) 14:45, 11 December 2013 (UTC)

I am very familiar with this subject. The confusion may result from the fact that prior to 2011-2012, the recommendations for the maximum daily dosage were higher than 40mg (I don't remember what exactly is was, and can't seem to find it now). However in 2011-2012, it was discovered that citalopram (Celexa), and the closely related escitalopram (Lexepro), could cause a dangerous heart arrhythmia (prolongation of the QT interval) in some patients. This was deemed especially dangerous for people with a condition called Long QT syndrome. It was determined that this possibility was dosage dependent and the "maximum daily dosage" of citalopram was reduced to 40mg and escitalopram to 20mg. See this FDA article for further information.--William Thweatt ^TalkContribs 16:31, 11 December 2013 (UTC)

Thanks everyone for all contributions although everything that's been said here is familiar to me. It is sort of daily routine but still I hoped that there might be a loophole. The issue frankly was not an attempt to increase the dosage of citalopram but to combine it with quetiapine which does have a modest (0.17) gravity toward QT prolongation. AboutFace_22AboutFace 22 (talk) 02:20, 12 December 2013 (UTC)

Distinguishing between wet and cold

I like my washing to be dry when I put it away but (given the low temperature of the region where I live at this time of year) I am unable to distinguish between cold and wet using my hands. Is this the same for everyone? --78.148.110.243 (talk) 19:55, 10 December 2013 (UTC)

Actually, never mind. I googled with a different string and found Wikipedia:Reference_desk/Archives/Science/2009_January_18#wet_vs._cold --78.148.110.243 (talk) 19:59, 10 December 2013 (UTC)

Oh wow...that was a good thread! Yes, it clearly is the same for everyone...and almost 5 years later(!), I still think my explanation is a good one. It would be interesting to perform the experiment I suggested - I wish I'd remembered to do it! SteveBaker (talk) 21:44, 10 December 2013 (UTC)

If wet and cold could not be distinguished by touch they would not be distinct concepts. Hold the material until it warms in your hand, and see if it is dry or cloyingly wet. μηδείς (talk) 04:41, 11 December 2013 (UTC)

It's more that something that is both wet and a little bit cold cannot easily be distinguished from something that's dry and very cold by touch alone. "Wet" and "cold" are obviously distinct concepts because touch is not the only critical factor here. We often have separate concepts for things that we can't distinguish with our senses alone - so while your conclusion is correct, your reasoning is entirely flawed.

The difficulty arises because we have no sensors in our skin that measure wetness. All of the sense organs are safely tucked away under a layer of dead skin cells - so we can't directly measure what's happening on the surface of our skin. There is no chemical detector that senses contact with air or water. What we do have is sensors that measure the temperature just beneath the surface of the skin. So when you touch something cold, heat travels from your hand into the object, reducing the temperature beneath the skin by enough that these sensors can detect it. It's not a great system...but it's all we've got. Hence (for example) you can't easily measure the temperature of something that's around room temperature by touch alone. Metals seem cold to the touch when plastics do not - even when they are at the exact same temperature. That's because metals conduct the body heat away more efficiently than plastics do - so the temperature where our thermal sensors are drops faster when touching metals than plastics. Metals feel colder than they really are - plastics and other good insulators like wood feel warm.

Consequently, the sense is reversed when things are warmer than body temperature and metals feel hotter to the touch than wood or plastic. A metal park bench, sitting baking in Texas noon sunlight feels like it's going to burn your skin right off. A wooden park bench at the exact same ambient temperature will be quite comfortable to sit on.

It's the same thing with wet versus dry cloth. Wet cloth is a great conductor of heat - so it pulls heat from your hand and feels cold. But touching dry-but-cold cloth produces the same initial effect. The only way to detect a difference is to hold the cloth for a longer amount of time. The wet cloth will continue to conduct heat away for quite some time - until the entire volume of cloth plus water has heated up to body temperature. But the surface layers of the dry-but-cold cloth will gradually rise to body temperature and start to feel warm (although the initial feeling will be cold). The poor thermal conductivity of the material will avoid the need to heat up the entire volume before you get that warm sensation.

So the trick here is to hold the cloth for a while and see how long it takes to get warm. The faster it warms up - the drier it is. Sadly, this way of feeling doesn't seem to be hard-wired into our brains, so it takes conscious effort. Worse still, you have to judge things backwards. Grab the cloth and hold it for a while - if it feels warm after (say) ten seconds - then it was just cold (but dry). If it stays cold then it was wet (but not so cold)...exactly the reverse of what you'd expect. This also brings up a kind of observer effect - the only way to tell whether something is cold-but-not-wet is to warm it up...thereby changing the very thing you were trying to measure.

Humans are easily fooled into thinking that the sensors that are a part of our body are somehow perfect - or at least much, much better than they really are. The only perception we have of the world is through those organs - so it's easy to imagine that the world "really is" as we sense it. This is more obviously true for sight, sound, taste and smell - but we imagine that we have the ability to use touch to sense the difference between wet and dry, hot or cold, metals versus non-metals - when we really don't. We can generally infer what's going on and get some measure of that - but there are always underlying assumptions that lead us astray. When we assume that two objects are both at the same temperature - but one "feels colder" - we infer that it's a good conductor of heat and therefore must be metallic...but if they really aren't at the same temperature, then that "ability" to sense metals versus non-metals is easily fooled.

The wet versus cold problem is a "touch-illusion" - just like the "optical illusions" that can fool our eyes because of their inadequacies. Auditory illusions are less common - but they do exist - as do taste and smell illusions. For example, we imagine that the "sweetness" taste sensation implies desirable foods because that taste implies that high calorie carbohydrates are present - when in fact the sense can easily be fooled by all sorts of non-nutritious chemicals such as we employ as artificial sweeteners to make food seem more desirable - or downright lethal poisons such as the Ethylene glycol found in some car antifreezes. We've come to use our deeply flawed "sweetness" detector as a measure of food desirability - when in fact, it detects nothing of the sort! SteveBaker (talk) 14:31, 11 December 2013 (UTC)

Another way to tell if fabric is wet is by it's weight. Wet clothes will hang straight down, while dry ones will "fluff up". StuRat (talk) 14:39, 11 December 2013 (UTC)

Yes, or you could warm a bit in your hand, then see if it feels dry or still cloying to your lips of fingertips. μηδείς (talk) 18:17, 11 December 2013 (UTC)

If you live in somewhere like the northern part of Great Britain or the Great Panhandle of Canada, then the solution is to use a traditional airing cupboard. In just a few days, your smalls will be dry enough to put away in draws. --Aspro (talk) 18:25, 11 December 2013 (UTC)

Are draws the same thing as joors? μηδείς (talk) 00:23, 12 December 2013 (UTC)

I have no idea what a joor is but the word is normally spelt "drawers" in the UK. --TammyMoet (talk) 11:33, 12 December 2013 (UTC)

It's a common pronunciation of drawer in the Philadelphia area, considered a shibboleth. Dream and drink are often pronounced as if they were jream and jrink. (I am not sure how widespread the pronunciation is.) μηδείς (talk) 18:54, 12 December 2013 (UTC)

• Not 10 minutes ago I was asked to determine whether false-leopard-fur comforters coming out of the wash were wet, or just cold! (One was still wet, the other two were just cold.) Small world. μηδείς (talk) 01:28, 14 December 2013 (UTC)

Inertial & Gravitational mass

I've been having a discussion with User:Zee99 about changes to the article on Inertia. I've read about the Brans–Dicke theory, and confess that I don't really understand it, but I've always thought that under all (other) current theories, inertial mass and gravitational mass should be exactly equivalent, even when observed from moving inertial frames. Zee99 thinks that this is not true. Which of us is correct? Dbfirs 20:56, 10 December 2013 (UTC)

I posted something at Talk:Inertia. -- BenRG (talk) 22:03, 10 December 2013 (UTC)

The idea that there is no "special" reference frame in physics is of course true. But from the perspective of useful definitions of "inertial mass" (or "rest mass" to use an old-fashioned term) - the reference frame of the object itself is a handy one to pick when defining these terms. In the frame of the object itself, inertial mass and gravitational mass are always the same - so it's convenient to just talk about "mass" - and for almost all applications of terms like "inertia", where velocities are small compared to c, we can greatly simplify the discussions by simply discussing "mass". That's important here because we need to make this article approachable to "ordinary" people for whom relativity is largely irrelevant and knowing why their big-assed SUV is going to crush my Mini Cooper like a bug is something they really should understand! (cross-posted at Talk:Inertia) SteveBaker (talk) 15:00, 11 December 2013 (UTC)

I agree that the discussion of relativistic observations of mass is probably not needed in this basic article, and I would be happy to remove all the recent additions. I still think that, even when a body is moving past at close to the speed of light, the inertial mass (as measured by the force required to accelerate it) and the gravitational mass (as measured by how it affects adjacent masses) will still remain the same (since kinetic energy also has a gravitational effect). Do we have an article that discusses this? Dbfirs 17:05, 11 December 2013 (UTC)