Hygrometry. y 
8. The principles of hygrometry being now my only objed, 
it would not be proper to ent^r into particulars on its pradical 
part ; but I (hall here mention for once, that the fteadieft hy- 
grofcopic fubftahces are fubjed to anomalies : for inftance, after 
an hygrofcope has remained fixed in water for many hours, if it is 
taken out* fuffered to dry a little, and then put again into water , 
it may fometimes happen to overpafs that point. In the 
fame manner, after an hygrofcope has been long fixed in the 
lime-veflel, it may happen alfo, that in taking it out only for 
a quarter of an hour, and putting it in again, it will move a 
little farther than it was before. Again, if in taking it out of 
the llme-vejfel^ where it had long remained fixed, it is put into 
water , and then back into the lime-vejjel , it may happen, that 
it will fix itfelf a little fhort of its former point, and never move 
thence, except by repeated great variations of heat ; but if, when 
it (hews that difpofition, it is taken out for a Abort time, and put 
in again, it will then attain its ufual point. This was the 
cafe in the iaft trial of my Jiandard . Laftly, the fame anoma- 
lies may take place at every other point of the fcale of every 
hygrofcope, only more or lefs according to the fubftances ; 
feme of which, for that reafon, cannot be ufed for pradicai 
hygrometry. 
9. Thofe anomalies of the fteadieft hygrofcopic fubftances, 
will probably prevent our ever having in the hygrometer an 
inftrument nearly fo exad as the thermometer ; and this I was to 
premile, that when i mention the refults of particular hygrofcopic 
experiments, it may be underftood, that they have only the 
degree of exadnefs that belong to their clafs. Luckily thofe 
anomalies are yet of no confequence for the great objeds of by - 
grology and meteorology ; the prefent ftate of hygrometry being fuf- 
ficient to excite on thofe objeds, queftions of great importance for 
natural 
