relating to Hygrometers and Evaporation. S3 



By means of his ingenious researches, and particularly his 

 valuable " Essays," Mr Daniell has contributed greatly to im- 

 prove and extend the science of hygrometry. But without 

 meaning at all to detract from the undoubted merit of these 

 labours, I am not convinced that his hygrometer is either the 

 most convenient and distinct, or even susceptible of being ren- 

 dered so. Whoever has attended to such instruments, must 

 have remarked, that the instant of incipient deposition is not 

 well defined. This defect, to be sure, is not so conspicuous 

 when the instrument is small, and the temperature changing 

 rapidly ; but if a cold liquid be put into a pretty large vessel, 

 along with a sensible thermometer, it will be found, that even 

 then the temperature of deposition cannot be ascertained with 

 much nicety ; and, of course, the uncertainty must be greater 

 with a small fickle instrument moving by starts. Besides, good 

 light and acute eyes are quite indispensable, simultaneously 

 to observe the fleeting temperature, and the corresponding but 

 ill defined commencement of the formation of dew. 



On the other hand, when we use a thermometer depressed by 

 the evaporation of water, as is the case with Professor Leslie's 

 hygrometer, this may be observed with the greatest nicety and 

 deliberation. A more legible indication is unnecessary ; and its 

 certainty and sensibility are placed beyond a doubt, by the ex- 

 act agreement of several such instruments. Nothing, indeed, 

 is wanting to remove prejudices, and give perfect confidence to 

 this simplest of all hygrometers deserving the name, but a 

 greater number of experiments by different observers. Even 

 supposing that some imperfection did attach to its theory, yet 

 more extensive observations could soon settle or correct this ; 

 but an infinite number of observations could not make the in- 

 stant of deposition well defiiied, whilst in the nature of things it 

 is otherwise. 



It is much to be wished, that accurate experiments were made 

 by different individuals, to ascertain the quantity of moisture 

 which can exist in the air at low temperatures. Authors of 

 great celebrity differ so widely on this head, that it is doubtful 

 if any of them be quite correct. To attempt to ascertain the 

 force of vapour at low temperatures by means of a column of mer- 

 cury, is out of the question ; because such a method is liable to 



