1 8 Mr. De Luc on 
which I ftudied in the beginning of my refearches in meteoro- 
logy ; but all that I concluded from my experiments and ob- 
fervations was, that we could not underftand thofe phenomena 
without fir ft having a fure hygrometer . This is the reafon 
why, foon after 1 had made my firft hygrometer , I expofed it 
in the open air in the country, fufpended very little above the 
grafs, from the morning of a fine day to the time of dew in 
the evening; the grafs grew wet, and the hygrometer remained 
at a great diftance from the point which had been fixed in water . 
I have related that experiment in § 91, of my firft Paper. 
28. When I had made hygrofcopes of various forts of flips ; 
for inftance, of different woods and of whalebone , cut acrofs 
the fibres ; of ivory and horn , reduced firft into thin tubes, 
and then cut in fcrew; and of quills , by cutting alfo in fcrew 
their barrels; I repeated, with thofe inftruments, my obfer- 
vations on dew ; and to give a (hort, but determinate idea of 
the phenomena I obferved, I (hall reduce them to fome gene- 
ral cafes, as indicated by one only of thofe hygrofcopes , that of 
quill , which, like all the others, is divided into xoo parts, from 
extreme drynefs to extreme moijlure . Thefe hygrofcopes were 
fufpended in the open air, three feet above a grafs-plat in the 
country,., ift Case. When a clear and calm evening fucceeds 
to a clear and warm day, the grafs frequently grows wet , 
though the above hygrofcope ftands many hours, and fometimes 
the whole night, between 50 and 55. 2d Case. If the dew 
increafes, fo that taller herbaceous plants and Jhrubs grow wet 
in fucceffion, the hygrofcope moves more and more towards 
moijlure and when it is come to about 80, plates of glafs and 
oil-paint alfo grow wet ; but at that period, neither metallic 
plates, expofed like the glafs ones, nor fome fhrubs and trees, 
are wet; and this alfo may laft whole nights. 3d Cafe. If the 
dew 
