Donovan — On a ComparaUe Hygrometer. 483 
moves from zero through 10% 20°, 30°, &c. ; the latter when it moves 
backwards from zero through 90°, 80°, 70°, &c. 
In order to obtain some knowledge of how this apparatus would 
act, I proceeded as follows : — A hygrometer, made according to the 
foregoing description, was placed under the glass receiver, on its plate, 
resting on several discs of spongio-piline impregnated with chloride of 
calcium and well dried. The arrangement was thus left for two days. 
The index had ceased to move, and indicated what is considered the 
point of extreme dryness. The hygrometer was then entirely immersed 
in water : the index immediately began to turn, and continued to do 
so quickly, until it had moved round the dial ten times, and a few de- 
grees over; the whole time occupied being 62 minutes. This was of 
course the point of extreme moisture. When taken out of the water 
and exposed to the air, it returned in 69 minutes, but stopped at the 
ninth round, the weather having become cloudy. 
The method of total immersion was used by De Luc in graduating 
his whalebone hygrometer, and by Leslie for the graduation of his 
ivory instrument. This method has been objected to as not represent- 
ing what really takes place in the atmosphere, which is the precipita- 
tion of water in the state of ultimate division, or as a vapour at the 
point of deposition. I do not perceive any practical difference between 
the two modes of expression : what is deposited is water in ultimate 
particles, and if these coalesce, rain or running water results, which is 
what I use for rapidly producing extreme moisture. "When a gut line 
has been exposed for a long time to saturated vapour, it is found covered 
with adhering globules of water ; total immersion could do no more ; 
the gut line will, in either case, absorb only to saturation. 
Whether there be, in the higher atmosphere, a region of absolute 
dryness is of no use to inquire ; but that there is a region of extreme 
dryness, we have the evidence of those who ascended in balloons, who 
reported that their hygrometers indicated an increasing siccity in pro- 
portion as they ascended, and to such a degree that paper became cor- 
rugated as if by fire, and thirst became intolerable. With such siccity 
we have nothing to do. In our experiments, we attain what we call 
extreme dryness by the employment of exsiccants ; but dryness so pro- 
duced is probably not absolute. I adopt therefore the two fixed points, 
extreme dryness and extreme moisture, both artificially produced, as 
the limits of my scale ; and as the representatives of the conditions 
which alternately exist in the heavens; and I assume the hygrometric 
range between them to be measured by the twisting and untwisting of 
the gut line, as shown by its index and graduated circle. 
A hygrometer, of whatever construction, can only indicate the at- 
mospheric moisture of its own immediate neighbourhood, which may 
be called its local atmosphere, and which is continually changing, 
either by the addition or subtraction of water in the state of vapour ; 
or by change of temperature or of pressure. 
R. I. A. PROC VOL. I,, SEE. II., SCJEXCE. 3 R 
