118 Mr. Colebrooke’s Meteorological Observations 
body, the depression of temperature is checked, and falls short 
of the point of dew. 
Accordingly it is found, that a thermometer, dripping wet, or 
wrapped in wet rag, or (bibulous) paper, exhibits depressed 
temperature, which continues to descend, until it arrives 
at a certain point, where it becomes stationary, and remains 
so for a while, until the surface of the instrument begins 
to dry, when the temperature slowly rises to that of the air 
around it. 
The limit of depression is, as before noticed, the point of dew, 
or that at which condensation of vapour takes place. The 
difference between this and the temperature of the atmosphere, 
shews the degree of dryness ; which is readily and conveniently 
observed by placing a separate dry thermometer near the wet 
one in the same exposure, 
The exposition must be such as allows full access to the 
air, which is the subject of observation ; in short, a free venti- 
lation: and, provided that is the case, a difference in the 
strength of the current of air is attended with no variation of 
result; other circumstances remaining unchanged. The blast 
of a bellows produces no alteration; nor does a gust or puff 
of wind, unless the temperature of the air, or its dampness, 
vary. 
With that simple hygrometer, observations were taken during 
my voyage over the Atlantic, in the latter part of the outward 
voyage and the whole of the returning one. 
This application of the thermometer to a hygroscopic pur- 
pose was first devised by Professor Leslie; but I had not a 
distinct recollection of his invention, when I put a thermometer 
to this use; and I have therefore retained my own exposition of 
the grounds on which I then proceeded; and the rather, as 
differing in some respects from his *. 
It is an instrument of much readier application than Mr. Da- 
niell’s hygrometer; and, as I apprehend, accurate, and sus- 
* Since this was written, I learn, from an Essay. by Mr. Ivory, that. the 
invention belongs to Dr. James Hutton. 
