webs from their tails so as to render themselves buoyant, and lighter than air. But why these apterous insects should that day take such a wonderful aërial excursion, and why their webs should at once become so gross and material as to be considerably more weighty than air, and to descend with precipitation, is a matter beyond my skill. If I might be allowed to hazard a supposition, I should imagine that those filmy threads, when first shot, might be entangled in the rising dew, and so drawn up, spiders and all, by a brisk evaporation, into the regions where clouds are formed: and if the spiders have a power of coiling and thickening their webs in the air, as Dr. Lister says they have [see his Letters to Mr. Ray], then, when they were become heavier than the air, they must fall.
Every day in fine weather, in autumn chiefly, do I see those spiders shooting out their webs and mounting aloft: they will go off from your finger if you will take them into your hand. Last summer one alighted on my book as I was reading in the parlour; and, running to the top of the page, and shooting out a web, took its departure from thence. But what I most wondered at was, that it went off with considerable velocity in a place where no air was stirring; and I am sure that I did not assist it with my breath. So that these little crawlers seem to have, while mounting, some locomotive power without the use of wings, and to move in the air faster than the air itself. 1
NOTE TO LETTER XXIII.
1 The appearance of the gossamer-covered fields will be familiar to all who live in the country. It seems clear that the "locomotive power" of the tiny spiders is due solely to the movement of the atmosphere. On the quietest days, if you will wet your finger and hold it up, you will find it grow sensibly cooler on one side than the other, and on that side is there a faint wind blowing. If you will then watch the spiders, you will see them shoot out long silvery threads, which will incline to leeward, and presently the spiders will let go their hold of the grass, and launch themselves into the air, floating away on the slightest movement of it.
