370 Mr Gordon on a Meteorological Journal for 1819, 
pend on the mechanical texture of animal or -vegetable substan- 
ces, and are subject, therefore, to such derangements, as must 
render their indications altogether unsatisfactory. The Diffe- 
rential Thermometer, applied to the purposes of hygrometry, is 
not liable to the objection now stated ; but as its indications are 
modified by temperature, it does not directly convey any satis- 
factory information respecting the hygrometric state of the air. 
An accurate hygrometer, therefore, was still a desideratum in 
meteorology, till the publication of the article Hygiiomete-y, in 
the Edinburgh Encyclopcedia^ where it has been supplied, not 
by the invention of a new instrument, but from contempora- 
neous observations of the thermometer and hygrometer, or sim- 
ply of two thermometers, one of them having its bulb covered 
with moistened silk or paper. In that profound and ingenious 
treatise, the author * has given a formula from Avhich may be 
deduced, in any given state of a wet and dry thermometer, the 
following interesting results. 
1st, The point of deposition, or that temperature at which the 
air, if cooled down, would begin to deposit moisture. 
2d, The absolute humidity of the atmosphere, or the quanti- 
ty of moisture contained in a given portion of air. In the fol- 
lowing tables this moisture is expressed in decimals of a grain 
Troy for every 100 cubic inches of the atmosphere. 
3d, The relative humidity of the air, complete dryness being 
denoted by 0, and complete saturation by 100. This result ex- 
presses the quantity of moisture which the atmosphere contains, 
in hundredths of what would produce complete saturation, at 
the given temperature. 
To facilitate the application of the formula, I have construct- 
ed from it a table, by which the above results may be found 
from simple inspection. It is hardly necessary to observe, that 
the whole is deduced from the depression of tenjperature, by 
evaporation from the moistened thermometer below that of the 
dry one. 
I have only to observe farther, that the quantity of rain is as- 
certained by a gauge placed in an open situation, six feet above 
the surface of the ground, and the evaporation by a basin sheltered 
from the sun, and nearly in contact with the thermometers. The 
* Adam Anderson, Esq^. F. R. S. E. rector of the Academy, Perth. 
