SUPPLEMENTARY REPORT ON METEOROLOGY. 95 
myself by experiment to be the fact. Hugi states (what is very 
conceivable), that the boiling point of water anticipates the 
barometric indications by some hours; but he states (what is 
more difficult to understand), that the boiling point of alcohol 
harmonizes with the latter*. 
Il].—Huvumipiry+. 
A. Hygrometers. 
164. No new hygrometer has been introduced of late years, 
so far as I am aware, at least no important novelty, but very 
considerable progress has been made in the right interpretation 
of results ft. 
165. By far the completest historical treatise which [have seen 
on hygrometry and hygrometers, is a learned thesis by Suer- 
man (different from the one already cited), entitled ‘‘ Commen- 
tatio de definiendd quantitate Vaporis Aquei in Atmospherd,”’ 
&c.§, which also contains good figures, and many pertinent 
original criticisms. 
166. As hygrometry essentially turns upon a right knowledge 
of the relations of heat and moisture, we may first observe, that 
a considerable number of attempts have been made (without new 
experiments) to express, by some simple formula, the relation 
between the temperature and pressure of vapour. The only 
experiments of any consequence that I am acquainted with are 
those of the Franklin Institute (America), conducted under the 
superintendence of Prof. Bache||._ They do not much surpass 
10 atmospheres, and even there the difference is 6° Fahr. be- 
tween them and M. Dulong’s results, a difference which does not 
seem to be satisfactorily accounted for. M. Dulong’s formula 
represents observations above 1 or 2 atmospheres better than 
those below ; and there is reason to think that, whether from the 
mode of conducting the experiments, or some other cause, there 
is some solution of continuity in the law which expresses the 
relation of density and pressure somewhere near the point arbi- 
trarily called the boiling point. 
* Naturhistorische Alpenreise, p. 16. , 
+ See last Report, p. 239, and Mahlmann, p. 129. 
t “Jam vero letior campus arridet quo recentiorum experimenta exponenda 
veniunt, qui, de vaporis natura longe certiores, multa simpliciorem tutioremque 
— quam przecedentes physici, ingredi potuerunt.”—Suerman, Commentatio, 
§ 45. 
§ 4to. Lugd. Bat. 1831, p.128. For this, too, I was indebted to the late 
Prof. Moll. 
|| Report on the Explosions of Steam Boilers. Philadelphia, 1836, p. 76. 
4] The formula which the American Committee adopt to represent their re- 
sults is (for Force corresponding to Degrees of Fahrenheit) e = (-00333 ¢ + 1)°. 
