How Spiders Fly

I was very much interested, a few days ago, in hearing a friend give an account of a manuscript she had seen, whieh was written by Jonathan Edwards when urne years old. It was an account of the behavior of certain small New England spiders, ttie manner they fly through the air, and the way to see them best. by getting into the edge of a shadow, and looking toward the sim. It is neatly and carefully written, and illustrated by little drawings very nicely done. The philosophical tendenties of the young writer already appear, for his conclusión as to the "final cause" of spiders and their flying is this: the little animáis are scavengers, and since, in New England, the prevailing winds are west, they are carried to the sea in their flight with whatever filth they have consumed.and so the land is cleanaed. Every one knows how, in sunny weather, the little creatures, standing on their heads, project from their spinnerets fine filamente of gossamer,which are caught by the breeze, and float off into the air, though still attached to the spider. When she preceives that the thread is long enough, and the pull of the wind sufficient, she releases her hold and nies away on her gossamer like a witch on her broomstiek; by watehing her chance, and lecting go only when the breeze is favorable, she is carried to her desired haven. Experimenta have been tried by placing the animáis on a chip floated in a pail of water. So long as the air waa in motion about them they were able very soon to escape f rom their island; but when a bell glass was placed over the pail, thus preventing air currents, they could not get from the island to the surrounding shore. But how does it happen that, on setting out for a voyage, the spider almost invariably ascends with her web and continúes to rise, until, by pulling in her thread, she reduces her floating power, and so comes down? 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 projeeting 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 Iets go her hold she and her thread are carried up partly by the action of this current. But this is not all; unless I am much mistaken, the action of the sun's rays on the thread itself and lts surrounding envelope of air is the main cause of its buoyancy. Air is nearly diathermanous, or transpiirent to heat, so that the solar rays, in traversing it, warm it only slightly. The spider's thread is not so, but in the sunshine warms up almost instantly, heating 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 warmed and expanded air taken together, are lighter than the same bulk of the cooler air around, and thus constitute a quasiballoon, on which the apider sails away. Of course, if this ia so, the poor creatures cannot sail much on cloudy days and I think, in fact, they do not. I have tried a few experiments to verify the idea, and so far as they go they all conflrm it. For instance, one day in the autumn of 188U, when the air was full of floating gossamer, and there was no wind blowing, I caught some of the filamente at the end of a little atick, to aee how they would behave. So long as I stood in the sunshine they streained straight upward, tugging with almost a breaking strain; as soon aa I stepped into the shadow of a building, they lost their spirit, and drooped 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 aocount for their behaviorj but once a cloud passed cross the sun, and they droeped then, just as they did behind the building. The same theory will explain the buoyancy of any minute partióles of dust or smoke. So long as the sun shines, they will absorb its rays, become warmer than the air, and surround themselvea with a buoyant envelope, which will carry them up, if they are not too heavy in proportion to their surface. But if the air ia still and the sun obscured, they will settle down near the earth, in the way we are all familiar with in muggy weather. Of course, if there is much wind, this will mainly control their movements, and neither their buoyancy in sunsbiae. nor their gravity in shadow, will be