$68 Appendix to M, Ramond*s Instrmtions, ^c. [Nov, 



site order to the augmentations and diminutions of humidity. 

 We conclude, therefire, that the effects of this cause are coun- 

 teracted by those of a cause so preponderating, that, after having 

 compensated the action of humidity, the excess of its own 

 influence extends yet further. 



(Instructions, p. 197.) — ^The hygrometer has not yet been of 

 any utility in the mensuration of heights, and there is little pro- 

 baoility that it can be introduced, not only because the correction 

 will be very small, but further because it will be very uncertain, 

 whether we consider the ignorance in which we are of the law 

 which the decrease of humidity in the column of air follows ; or 

 the extreme difficulty, if not impossibility, of eliminating this 

 law in the result of experiments always made at the surface of 

 the earth ; that is to say, at the very source of those influences 

 which modify partially and irregularly the humidity of the atmo- 

 sphere. Saussure thought thus, and we are of the same 

 opinion. The mean value of the humidity comprised in the 

 constant coefficient and the factor belonging to the temperature, 

 will occasion less error than a theory ill supported by observa- 

 tions will do ; and these errors after all are of such small conse- 

 quence, that they are not worth the trouble of a calculation, 

 which will only cause a variation in the chances, even if it do 

 not multiply them. 



Such are the opinions of M. Ramond on the subject of a 

 correction for the different state of moisture in which the air 

 maybe at the two stations; and to his great practical experience 

 we must doubtless pay the highest deference ; at the same time 

 it becomes necessary to recollect the great improvements which 

 have taken place in the science of hygrometry subsequently to 

 the date of the methods described and used by ourdistmguished 

 author. Hence several philosophers of the present day have 

 not considered it undeserving attention to examine into the 

 propriety of introducing the correction in question. Some 

 observations seem to indicate an effect by no means inconsider- 

 ate due to the presence of vapour. We may cite the instances 

 of Mr. Greatorex's observations on Skiddaw (Phil. Trans. 1818, 

 Part II.), in which a considerable discrepancy appears to have 

 been connected with some changes in the hygrometric state of 

 the air. The measurements of Messrs. Herschel and Babbage 

 at Staubbach (Edinb. Phil. Journ. No. 12), seem also to have 

 been affected by the same cause. I merely refer to these cases, 

 however, in order to observe in general that should more 

 extended observations show the necessity of an application of 

 the hygrometer in barometrical operations, the formula above 

 investigated will easily admit of the introduction of a variable 

 factor for this correction, instead of the mean value at present 

 involved in the constant coefficient, and slightly modified by 

 the variation of temperature. 



An excellent method of ascertaining the elastic force of the 



