376 Mr. Moyle on the Height of the Barometer. [May> 



Article X. 



On the Height of the Barometer. By M. P. Moyle, Esq. 



(To the Editor of the Annals of Philosophy.) 



DEAR SIR, Hchton, April 15, 1823. 



A proper collection of accurate barometrical observations 

 would give us a correct idea of the height of the different places 

 where they are kept above the level of the sea, as well as serv- 

 ing other useful purposes ; but unless every necessary correction 

 is made, this cannot be obtained. From the few observations 

 that I have made on those tables which have appeared in the 

 Annals, this consideration does not seem to have been properly 

 meditated. It may not, therefore, be unacceptable to such of 

 your correspondents to make a few remarks on this particular 

 point, and thereby prove its necessity. 



It is well known that mercury expands by heat, and contracts 

 on the application of cold. Hence the height of the mercurial 

 column in the barometer is affected not only by the pressure of 

 the atmosphere, but by the temperature, and the attention of 

 meteorologists ought to be more fully drawn to this considera- 

 tion to render their tables of the greatest utility. 



The standard temperature for observation is agreed on all 

 hands to be 32° of Fahr. ; consequently if made at a time when 

 the thermometer stands above or below this point, it must of 

 necessity indicate a higher or lower range respectively ; and to 

 prove the extent of error arising from the neglect of this circum- 

 stance, let us suppose that the height of 30 inches of mercury 

 is taken when the attached thermometer stands at 72°; this 

 would give an excess above what it would be at 32° of more than 

 -j-'^ths of an inch from the mere expansion of the mercury. 

 !No wonder then that errors arise in our calculations. 



General Roy found that the expansion of one inch of mercury 

 in the barometer tube at 32° was -0001127 : hence to reduce the 

 observed height of the mercury to what it would be at 32°, 

 becomes an easy matter; but for the greater facility, 1 have 

 constructed the following table, which represents the expansion 

 of one inch of mercury for its corresponding temperature. It 

 extends from 32° to 150° beyond which it is not probable that 

 any observation will be made. 



In order to obtain the exact temperature of the mercury, the 

 observation should be made by a thermometer attached to the 

 frame of the barometer, that it may warm and cool along with it. 



