112 PHILOSOPHICAL TRANSACTIONS. [aNNO 17 9^ • 



he observed the correspondent changes of those instruments, and, by combining 

 the results of his experiments, he reduced to regular series the correspondent 

 motions of the 2 instruments by equal quantities of evaporated water. Having 

 confined himself to that only class of experiments, whicli could not discover the 

 difficulties of his attempt, he thought himself warranted to draw from them the 

 following conclusions. 1st, That the degrees of moisture in the inclosed me- 

 dium were nearly proportional to the quantities of water evaporated in the vessel; 

 and that, consequently, the ratio observed between those quantities and the 

 march of his hygrometer, could be considered as giving immediately the march 

 of the instrument correspondent to moisture itself; which, according to our com- 

 mon opinion, is a certain quantity of aqueous vapours spread in the medium. 

 2dly, That when no more water could evaporate in the vessel, the inclosed me- 

 dium was arrived at extreme moisture ; and that, consequently, the point in- 

 dicated at that time on his hygrometer, was to be the limit, of its scale on that 

 side. 3dly, That having, from those experiments, a probable determination of 

 the expansions of the hair by successive equal quantities of moisture, in beginning 

 from the point where this is null, and ending at its extreme, his instrument could 

 not differ essentially from an absolute hygrometer. 



These conclusions were very natural in the state of M. de Saussure's experi- 

 ments ; but before their publication Mr. D. had gone over a great field of hygro- 

 scopic phenomena, in which the hair, and a close vessel, had a share ; and there- 

 by seeing the objects in another light than M. de Saussure, he doubted of his 

 conclusions, and he procured 3 of his hygrometers, in order to examine them on 

 some particular points. It was after that immediate verification of his conjectures 

 concerning Mr. S.'s instrument, that Mr. D. settled the following conclusions, 

 very different from those above. 1st, That moisture, or the quantity of vapour 

 spread in the medium itself, does not increase in an inclosed space in proportion 

 to the quantity of water evaporated in it ; because of an increasing, but unde- 

 termined, part of that water being deposited on the sides of the vessel ; and that, 

 consequently, M. de Saussure's experiments could not afford the determination 

 of a real hygroscopic-scale. 2dly, That the circumstance considered by him as a 

 sure sign of extreme moisture existing in the inclosed medium, namely, the 

 maximum of evaporation in the space, has only that effect when the temperature 

 is very little above 32"; but that, by successive increases of heat from that point 

 moisture recedes further and further from its extreme ; or from the point where 

 no more vapour can be introduced in the medium without an immediate precipita- 

 tion ; thovigh at the same time, there are successive increases in the quantity of 

 vapour, and thereby a constant maximum of evaporation correspondent with the 

 actual temperature. 3dly. That, in approaching to extreme moisture, the hair 

 hygrometer becomes stationary, and afterwards a little retrogratle, in which march 



