294 HYGROLOGY, AND ITS CONNECTION WITH METEOBOLOGY. 



Scale. remains there. This point I have called 100, and the scale is 



divided into 100 parts. 

 Inquiry after g. Another important object treated in the same paper, 

 stance for hy- ^"^ which occasioned me much labour, was, of what substance 

 grometers. the hygrometer should be constructed. On this particular 

 point I related a long series of experiments, occasioned by the 

 first results I obtained by trying many kinds of animal and 

 vegetable substances : some of which could be used in thin 

 threads, tovn in the length o( their Jlbres ; and also in thin 

 slips cut across the Jlbres. Now, I found, that when used in 

 the length of the fibres, their lengthening by moisture decreased, 

 and at last they were even shortened, while the same substances 

 cut across the fibres continued to lengthen : which at first em- 

 barrassed me very much^'. 

 Exp. to find 9- I could not decide immediately from these observations, 



whether sub- -whether the substances taken in length continued to imbibe 

 stances cut . , , , i , , ■ 



lengthwise im- moisture, whiJe, however, then- length was decreasing ; and 



bibed moisture jn order to ascertain this necessary point, I contrived a vessel, 



ing. ' described in the same paper. In that vessel I enclosed together 



several pairs of hygrometers, made of the same substances ; in 



one, it was used in the length, and in the other across the fibres; 



and a beam, indicating the 500th part of a grain, to which I 



Reasosswhy * The reason of the difference in the successive expansion by moisture 



the substance of the same fibrous substances, taken in the length and across their fibres, 

 aero th ^ proceeds from the nature of these substances. The main ^/,rfs in their 

 ofsXa. length are unkedhy Jihils, which are seen when we split these bodies. 



These small^ojTS form with the larger ones a sort of ??i<'.s7jev, similar to 

 those of a ?ie<-. The first effect of moiature is oi\ ihe longitudinal ^ores, 

 yNhlch. It lengthens ; but when it penetrates the meshes, it tvvlens them, 

 and thus shortensi the body ; as the length of a net is lessened by stretch- 

 ing it across. Moisture therefore acts in two opposite ways on the Jibrous 

 substances taken in length, differently in its progress on the same sub- 

 stance, and differently also in different substances. And besides, the 

 whole lengthening is very small in all of them. Now, one of these effects 

 is suppressed by taking the same substances across the fi'ircs, namely, 

 that which acts on the length of the latter ; there remains only that 

 which acts on the breadth of the meshesy which, if not absolutely pro- 

 portional to the increase of moisture, is never in an opposite sense. Be- 

 sides, there is a great gain with respect to the extent of the lengthening^ 

 and therefore of the degrees of the hygrometer ; for instance, a s^?p of 

 whalebone, by passing from extreme dryness to extreme moisture, increases 

 nearly one ninth in length. 



suspended 



