406 



VHILOSOPHICAL TRANSACTIONS. 



[anno 1778. 



rules and observations with my own; and am not more pleased than surprised at 

 the general correspondency of our results, which carries with it the appearance 

 of one and the same series of experiments, rather than of distinct observations 

 made with different instruments, in different countries, and by different persons. 

 That the standard temperature of zero on the scale of the thermometer should 

 be found by each of us to tall in the same point to within 4- of a degree is, I 

 think, truly surj)rising; and I doubt not will evince to Mr. de Luc the strong 

 probability there is of the necessity of correcting his rules. But though in this 

 essential and fundamental part of the inquiry we agree, there are however some 

 little circumstances in which we differ; and it is the subject of this letter, sir, 

 to point out to you the degree of our differences. The 2 chief causes of our 

 differences are, the expansion of quicksilver and the expansion of air. I shall 

 begin with the equation for quicksilver. The mean temperature of ordinary 

 barometrical observations will generally be found to lie between 40° and 70" on 

 Fahrenheit's thermometer: now the mean expansion in this range, according to 

 your observation, is ,0323 inch on a column of 30 inches for 10° of heat; by 

 my table it is only ,0304 inch: the difference ,00 1 g inch is equal to about 20 

 inches in the result of the height, when the temperature of the 2 barometers 

 differs 10°, and this may reasonably be expected only in a height of 3000 or 

 4000 feet. In an observation on Mount Etna, one of the greatest accessible 

 heights in Europe, the diffisrence of temperature at the top and bottom might 

 amount to 30", and this would occasion a difference of about 5 feet, which may 

 be reckoned inconsiderable in a height of ll,00o feet. In fact, in an observa- 

 tion on this mountain by Mr. Dessaussure it amounted to only 3-l feet. I may 

 add, that your equation makes the computed height less than mine. 



I proceed to the expansion of air. Your equation is various according to the 

 circumstances, the difference therefore of our results will, according to the cir- 

 cumstances, be various. The following table will give the quantity of this dif- 

 ference, viz. it shows how much your result is -f- or — mine on 1000 feet, 

 according to different pressures of the atmosphere and different tomperatures. 

 The first column to the left hand contains the mean heat of the colunm of air, 

 between the 2 barometers; the figures in the horizontal line at top are the mean 

 height of the 2 barometers, or mean pressure of the atmosphere; the common 

 point of meeting in the different columns gives the difference of our result in 

 feet, according to the respective circumstances. 



Mean height of the two barometers in inches. 



29 



•28 



27 







(J 



() 



o 



4- 'J I _i 1 -4. 

 + + I I -4 



O 

 - 1 



-•2 - + 



20 





 - 9 

 -17 



-25 

 -33 



