Barometer applied to the Measurement of Heights. 365 



ion or 4. In common chamber barometers, the lengths 

 of the columns are read off to a 0,01 of an inch, and in the best 

 to the j| 7 or a 0,001 of an inch. 



The best time for taking observations with the barometer for 

 the purpose of calculating heights is during settled weather and 

 at midday. Observations taken in the morning or evening are 

 much more liable to be erroneous on account of ascending and 

 descending currents of air which take place at these times. 

 Moreover a course of continued observations is more likely to 

 lead to accurate results than single observations. By means of 

 accurate registers of the barometer, the difference of level of 

 places, however remote from each other, and their elevation 

 above the ocean, may be ascertained with a considerable degree 

 of precission. It is found, for example, by numerous and care- 

 ful observations in different parts of Europe, as stated by Biot, t 

 that the mean height of the barometer at the level of the ocean 

 is 0,7629 of a metre, the temperature being 1 2,8 of the centes- 

 imal scale. This, reduced to English inches, becomes 30,035, 

 and 1 2,8 of the centesimal scale corresponds to 55 of Fah- 

 renheit's. Now the mean of 22 years observation, three obser- 

 vations a day, at Cambridge, N. E. gives for the height of the 

 barometer at the same temperature, 29,9974 Applying the for- 

 mula to these observations, we shall have the following result, 



30,035 .... log. 1,47763 

 29,997 . . . log. 1,47708 



0,00055 



Multiplying by 10000, or which is the same thing, removing the 

 decimal point four places to the left, we obtain, for the approxi- 

 mate difference of level, or the elevation of the place of observa- 

 tion at Cambridge, above the ocean, 5,5 fathoms or 33 feet. 



Now the mean temperature at Cambridge, as ascertained by 

 corresponding observations during the same period is 48,8 ; and 

 if we take the temperature at Paris as the mean temperature of 



t Precis sur la Physique, vol. 1, p. 194. 3d. Edition. 

 t Memoirs Am. Acad. vol. iii. p. 386. 



