2 PHILOSOPHICAL TRANSACTIONS. [aNNO ITQI- 



water has a fixed limit, proceeding from a final resistance of their pores, to be 

 more dilated by the introduction of water. Consequently, their utmost expan- ' 

 sion is a true sign, that moisture is extreme in them ; which point cannot be ex- 

 ceeded. But the proposition extended further : Mr. D. had said, that water was 

 the only certain means of obtaining immediately the point of extreme moisture 

 on hygrometers ; which is a most important question, both of hygrometry and 

 hygrology, which remains to be examined. Mr. D. then has a dissertation on 

 the maximum of evaporation, and its correspondence with the maximum of 

 moisture in a medium. After which he proceeds to 2 distinct classes of hygros- 

 copes. These are such as consist of slips and shreds. The slips, consist of very 

 thin and narrow laminae cut across the fibres of vegetable or animal substances, 

 either in their natural or artificial breadths, as boards, or by reducing natural or 

 artificial thin tubes of them into helices. By threads, he means the same kinds 

 of substances taken lengthwise, either from their being naturally in thin threads, 

 or by reducing them to that state, in tearing from them thin fasciculi of fibres ; 

 which operation is easy in some, as hemp, whalebone, and gut, but very diffi- 

 cult in others, as quill and some sorts uf wood. Between these 2 different di- 

 rections of the same materials, as might be expected, were found great irregu- 

 larities, and many contradictory effects. Thus, hemp and gut have only a very 

 little retrogradation ; their greatest difference from the slips consisting in their 

 being stationary, while the slips have still great motions. But when these same 

 threads are twisted, they acquire a very sensible elongation beyond their point of 

 extreme moisture succeeded by retrogradation. From several trials made in 

 twisting these threads more and more, it seems not impossible, if some difficul- 

 ties were completely prevented, that they might be brought to such a state, as 

 to have their point of extreme dryness coincide with that of extreme moisture; 

 by which means, in the progress of moisture from one extreme to the other, 

 they would move first in one direction with decreasing steps, then in the opposite 

 direction by increasing steps ; the whole however with great irregularities. Here 

 then we see two opposite effects of moisture ; one which lengthens the fibres ; 

 the otlier which, bv swelling the twisted strings, shortens them ; and we see 

 those effects follow different laws, from which is produced a retrogradation that 

 we may change ad libitum. If, then, moisture, in acting on vegetable and 

 animal threads, natural and artificial, produces on their length two opposite 

 effects ; one of which, small at first but increasing gradually, compensates at 

 some period the other which is first visible, and surpasses it afterwards, sooner 

 or later, according to the nature of the threads ; it is evident, that they cannot 

 be proper for the hygrometer ; since, from the indication of some of them it 

 might sometimes be concluded, that moisture changes in one sense, while it 

 really changes in the contrary sense ; and from some others, that moisture is 



