4]4 PROFESSOR FORBES ON THE DETERMINATION OF HEIGHTS 



as I believe can be got in any other way. The mass of the water and also of the 

 thermometer favours this. 



But I had a farther test of the exactness of the arithmetical progression 

 above established, and that as severe as could well be proposed. It was to com- 

 pare DE SAUSSURE'S observations on Mont Blanc and the pressure there ob- 

 served with the result of my formula. But first, it was necessary to correct the 

 zero point of his instrument, and to render it comparable to mine. DE SAUS- 

 SURE'S boiling point, 80 of REAUMUR, or 212 of FAHRENHEIT, was adjusted at 27 

 French inches, or 2H.777 English. 



At that pressure my thermometer (AciE) shews 210.58 F. DE SAUSSURE'S 

 stood, therefore, 1.42 F. higher than mine. Now, on the top of Mont Blanc, the 

 barometer stood at 17.133 English inches. 



The boiling temperature by DE SAUSSURE was . . 187.234 Fahr. 

 Reduced to ADIE 185.814 ... 



But the boiling point of ADIE'S thermometer, with the baro- 

 meter at 30 inches, is 212.62 



Subtract 185.81 



At Mont Blanc, below boiling point at 30 inches, 26.7l 



By GALBRAITH'S Tables, .... 30.000 inches = 29228 feet. 



27.133 = 14593 



Height unconnected for temperature, 14635 

 Now, by the proportion found empirically above, 



Height uncorrected for temperature = 26.71 x 549.5 = 14677 feet, a coin- 

 cidence really surprising. 



I have already stated, that DE SAUSSURE found DELUC'S formula to conform 

 accurately to his observation on Mont Blanc. It may therefore be concluded, 

 that DELUC'S formula and mine agree closely. In fact, if we take its conversion 

 into English measures, as given by Dr HORSLEY,* 



99 



899^ log z 92.804, 



which gives the boiling point, in degrees of Fahrenheit, reckoned from 32, z 

 being the height of the barometer in tenths of an English inch, we find that this 

 gives 



544.7 English feet of ascent for 1 Fahr. 



Practically, I consider it sufficient to find the difference of height, in feet, be- 

 tween two stations, to multiply the difference of the boiling points by 550, and 

 then correct as in barometric observations for the temperature of the air. 



* Phil. Trans., vol. Ixiv., p. 226. 



