174 Analysis of Scientific Books. 
suring both the elasticity and weight of the humidity in the at- 
mosphere, by his ingenious experiments on the force of vapours, 
and his theorem of their mixture with gases. Mr. Daniell has, 
finally, given the principle of Le Roi, and the tables of Dalton, 
the utmost precision, facility, and generality of application, by 
the invention of his hygrometer *. 
Dr. Black’s researches on the absorption of heat during 
the evaporation of liquids, suggested to Saussure and Hutton, 
another hygrometric measure, viz., the depression of a thermo- 
meter having its bulb moistened and exposed to a current of 
air. This method furnishes a convenient hygroscope, which 
Professor Leslie has modified by using an air thermometer on 
Van Helmont’s construction, instead of one with mercury or al- 
kohol. Of this change, however, we cannot perceive the 
advantage ; for two sensible mercurial thermometers, with one 
of the bulbs wrapped in dry and the other in moist mus- 
lin, both supported in one frame, will not only give results 
of sufficient delicacy, but are at the same time expressed 
in a language far less arbitrary than that of his scale. Com- 
mon thermometers have also the advantage of perfect por- 
tability and are still applicable to any other purpose, while Mr. 
Leslie’s hygroscope is easily deranged by carriage, and is at 
any rate fit for nothing else. 
The principle of Le Roi, as developed by Dalton, and ap- 
plied by Daniell, is further capable of determining the evapora- 
ting force in the atmosphere ; or of shewing the quantity of 
moisture which can rise from a given surface, in vapour, ina 
certain time. The same thing has been attempted in many 
ways; most of them, however, more or less complicated with 
the radiating power as to caloric, possessed by the body which 
contains the water. Thus if water be exposed in a clear day to 
the atmosphere in two shallow capsules, one of glass, and the 
other of silver, but both of the same size and shape, it will be- 
come colder in the glass, than in the metallic vessel, and will 
therefore evaporate more slowly in the former than in the 
latter. 
The writer of the curious treatise before us, is well known 
to have devoted a great deal of study to the relation of heat 
and moisture, the title indeed of a small tract published by 
him some years ago. Like that disquisition, the one now before 
us is remarkable for the prominence given to the description 
and praise of the author’s own contrivances, which are after all 
merely the varied aspects of Dr. Black’s proposition with regard 
to the absorption of caloric. But Mr. Leslie scarcely deigns to 
allude to any of his philosophical contemporaries to whom me- 
teorology is primarily indebted; nor do we find the slightest 
* Journal of Science and the Arts., vill. 298. 
