ON THE ELECTmC COLUMN. §57 



comparatively to the measured heights were equal ; after 

 which the causes of these anomalies became the object of 

 my researches. 



One of the principal means by which I had considerably Expansion by- 

 reduced the former great anomalies in this ^measurement, J^^^"°^^j[^^_'' 

 which had appeared unconquerable, had been by intro- count for 

 ducing an equation for the differences of expansion of air ^ "^ 

 produced by heat in the atmosphere. Considering therefore 

 this effect of heat, which, by its increase, diminishes the 

 pressure of columns of air of a determined height, under 

 the %^n\e pressure of superincumbent air, as indicated by 

 the height of the barometer at the tipper station ; and con- 

 necting this circumstance with the idea, that the cause of 

 heat is an expansible fluid, immely Jreefli'e, which occupies 

 in air a space without any sensible addition to its mass ; I 

 concluded, that some other^wic?, for which we had not yet 

 a test, as we have by the thermometer for the former, might 

 be the cause of the above remaining anomalies. 



This general conclusion brought into my mind the aqueous The presence 

 vapour, of which I knew that the 5/)ecj^coTd«ifi/ was much "^"queous va^ 

 less than that of air ; and supposing at that time, as is ftse/f^"^* 

 commonly thought, that its accumulation in the atmo- 

 sphere was the cause of rain, I conceived that the difference 

 of its quantity ni different times must be very great, and 

 that this ujight be the cause, or at least one of the causes of 

 the anomalies I had in viev/. 



The same consideration led me also to a S3'stem concerning and thestateof 

 the remarkable, though not constant, correspondence of the meter, ^ 

 variations in the sedentary barometer with rain and flne 

 weather I as the same fluid, the abundance of which in the 

 atmosiphere dimini hes \ts pressure upon the barometer, was 

 supposed to produce rain. Having published this system in 

 the above mentioned work, it obtained much approbation 

 among natural philosophers, because no satisfactory expla- 

 nation had been yet given of the above connexion of phe- 

 nomena ; howevei'T did not intehd to give it a full assent, 

 till i had succeeded in the construction of a true hygrometer ', 

 judging already, that, without such an instrument, nothing 

 could be determined with any certainty, concerning the 

 modifications of «'G]9ora/e<i it'f/ier in the atmosphere. This 

 Vot. XXVII.— Dkc. 1810. S judgment 



