HOW SPIDERS FLY.

Waikato Times, Volume XIX, Issue 1565, 15 July 1882, Page 6

HOW SPIDERS FLY.

I was very much interested a fow days ago, in hearing a friend give an account of a manuscript she had seen, which was written by Jonathan Edwards, when nine years old. It was an account of the behavior of certain small Now England spiders, the manner they fly through the air, and the way to pee them best, by getting into the edge of a shadow, and looking towards the bud. It is neatly and carefully written, and illustrated by little drawings, very nicely done. The philosophical tendencies of the young writer already appear, for his conclusion as to the "final .cause" of spiders and their flying in this : the little animals are scavengers, and since, in New England, the prevailing winds are webt, they are carried to the sea in their flight with whatever filth they have consumed, and so land is cleansed. Every one knows how, in sunny weather, the little creatures, standing on their heads, project from their Bpinnerets fine filaments of gossamer, which are caught by the breeze, and float off into the air, though still attached to the spider. When she pcrcoives that the thread is long enough, and tho pull of the wind .sufficient, she releases her hold and flios away on her gossamer like a witch on her broomstick; by watching her chance, and letting go only when the breeze is favorable, she is carried to her desired haven. Experiments havo been tried by placing the animals on a chip floated in a pail of water. So long as the air was in motion about them they were able very soon to escape from their ibland: but when a bell glass was placed over the pail, thus preventing air currents, they could not got fiom the island to tho surrounding shore. But how docs it happen that, on setting out for a voyage, the .spider almost invariably ascemh with her web, and continues to rise, until, by pulling in her thread, she reduces her floating power, and so comes clown ? Spider- web, in and of itself, is not lighter than air, how, then, is its buoyancy to be explained ? In two ways, I think. When the sun is shining, every projecting object, like a twig or stick, absorbs heat more rapidly than the air, becomes warmer than the air, and thus acts like an independent source of heat in generating an ascending current, so that when the spider lets go her hold she and her thread are carried up partly by the action of this current. Hut this is not all ; unless I am much mistaken, ml ion of ths sun.-, r«y\ on the thnud ihclf and its .surrounding envelope of air h the main cause ot its buoyancy. Air is nearly diathermanoiis, or transparent to heat, so that the solar rays, in tiavcrsing it, waun it only slightly. The spider's thread is not so, but in 'the sunshine warms up almost instantly bearing the air in immediate contact with it ; and then, although the spider thread alone is heavier than air, yet the thread and the adhering envelope of wanned and expanded air taken together, are lighter than the same bulk of cooler air aiound, and thus constitute a quasiballoon, on which the spider sails away. Of couisc, if this is so, the poor creatures cannot sail much on cloudy days and I think, in fact they do not. 1 have tried a few experiments to verify the idea, and, so far as they go, they all confirm it. For instance, one day in auttmn of ISSO, when the air u as full of gossamer, and theic was no wind blowing. I caught some of the filaments at the end ot a little stick, to see how they would behave. So long as I stood in the sunshine they streamed straight upward, tugging with almost a breaking strain ; as soon as I stepped into the shadow of a building, they lost their spirit and dropped abjectly ; the moment I put them in the light again they resumed their buoyancy. It is of course possible that in the shade there were local downward air currents to account for their behaviour ; but once a cloud passed acro&s the sun, they dropped then, just as they did behind the building. The same theory will explain the buoyancy of any minute particles of dust or smoke. So long as the sun shines they will absorb its rays, become warmer than the air, and surround themselves with a buoyant envelope, which will carry them up, if they are not too heavy in proportion to their surface. But if the air is still and the sun obscured, they will settle down neat the earth, in the way we are all familiar with in muddy weather. Of course, if there is much wind, this will mainly control their movements, and neither their buoyancy in sunshine, nor their gravity in shadow, will be particularly noticeable. — liodon Journal of Chemistry.