250 ^ Jordan o« the Floatage vf 



the particle. When, by the accession of others, the particles 

 are increased in their dimensions, the attractions of the air 

 which sustain, and the weights which precipitate the drops, 

 increasing in the before-mentioned proportions of the squares 

 and cubes of the diameters, the drops quickly increase to sizes 

 no longer sustainable in air, and immediately are precipitated. 

 When, however, the union of particles is just beginning, and 

 the new masses, in a state very near to solution, possess suf- 

 ficient tenuity, floatage is produced and clouds are formed, and 

 driven by winds through the atmosphere. 



In the same manner, if the particles of any body, heavier or 

 not than water, are, by any process, even the inverse of that 

 just stated, by diminution from larger parts, not as in the 

 preceding case by increase from dissolved parts, is made to 

 arrive at a sufficient degree of tenuity, they will be as capable 

 of floatage in the atmosphere as the smallest drops of water. 

 The filaments of spiders possess this tenuity. They possess it, 

 together with the power of sustaining the additional Aveight of 

 the spider, when of lengths sufficient for the purpose. The 

 spider, standing on a convenient station, continues to emit its 

 filament until it is felt to be capable of raising and does raise 

 him from his place, and then he abandons himself to its convey- 

 ance. Should this be done rather prematurely, he descends 

 wherever it may be, lengthens his filament, and mounts again. 

 That this is the case, appears from what I observed and have 

 stated ; and that the tenuity of the filaments is an adequate and 

 existing, and therefore a true cause, the calculations of 

 Lewenhoek show. He found that one hundred diameters of the 

 filaments of grown spiders were only equal to the diameter of 

 the hair of a man's beard, and the quantity of matter in a 

 filament therefore equal to 3o,Vt)o P^""*^ o^ ^'^^ quantity of 

 matter in the hair. Of young spiders, estimated to be severally 

 equal in size to -j J^y part of a grown spider, and their filaments 

 to be consequently in the same proportion, he estimated the 

 quantities of matter contained in each filament to be only the 

 4.TT(TW,oiTT7 P^i't of that contained in the hair. 



The action of the sun between the tropics, by heating and 



