Mr Foggo on the Dew-Point Hygrometer, S^ 



De Luc, the performance of all of which has only rendered 

 more striking the value of some convenient and effective plan 

 of reducing M. le Roi's method to practice. 



The manner in which Mr Daniell has resolved this problem 

 by an elegant application of the principle of the cryophorus 

 is too well known to require any description in this place. 

 By this contrivance, and still more by his popular exposition 

 of its principles and uses in many important researches, Mr 

 Daniell has given to the science of hygrometry an almost un- 

 hoped for degree of precision, and has certainly inspired fresh 

 energy into the study of almost every other branch of meteoro- 

 logy. I willingly add my tribute to the applause bestowed 

 on this ingenious adaptatpii of a beautiful philosophical prin- 

 ciple to the purposes of practical science, which, however, 

 forms only a small part of Mr Daniell's very successful and me- 

 ritorious labours. I think very few will call in question Mr Da- 

 nieirs opinion, that he has thus introduced a mode of ascertain- 

 ing the required data^ so easy and nice of application as to 

 leave no excuse for those who, at the expence of candour and 

 professional honour, intrude on the public the abortions of 

 their own dreamy lucubrations. By introducing the frigo- 

 rific power of vaporization from a volatile liquid, I conceive 

 that Mr Daniell has secured almost all the credit likely to be 

 acquired from the subject; and having acknowledged this 

 much, I may enter with more freedom into the discussion of the 

 merits of some suggested improvements upon his instrument. 



Mr Daniell thinks he has demonstrated that his hygrometer 

 is susceptible of no improvement. Without stopping to in- 

 quire on what the demonstration is founded, I shall quote some 

 remarks on his perfect instrument from a correspondent* who 

 unites a thorough knowledge of meteorological science to a pro- 

 found acquaintance with every other branch of natural history. 

 " It is a beautiful result, that, by applying ether at one ball, 

 such a degree of cold is produced as to effect a depression of 30* 

 or 40° in the other, notwithstanding that the vapour, in pass- 

 ing from one to the other through a long tube which remains 

 of the temperature of the air, will be heated up to this tern* 



* The Reverend Mr John Macvicar, Dundee. 



