56 Description of the Mountain Barometer, 



actly as the height of the mercury; that is, at 21 inchc? it 

 is 'OOiJl, at 22 inches '00^2, and so on; so that the height 

 in inches is the number of ttni thousanchhs oF expansion for 

 one degree of Fahrenheit. This is very easily remembered. 

 The expansion for any other number of degrees is in propor- 

 tion to the degrees themselves ; that is, for two degrees it is 

 twice as much, for ten degrees ten times as much, and so 

 on. Take therefore the difference oF height of the attached 

 thennomelers at the two stations, and multiply the expan- 

 sion for one degree at the coldest barometer (which will al- 

 most always be the one at the highest station) by the num- 

 ber of degrees of difference between the heal of the two ba- 

 rometers, and add the quantity to the observed height of the 

 coldest barometer, and it is corrected for the expansion of 

 the mcrcuj-y by the heat of the instrument. 

 An example will make the whole clear. 



Inche-'!. Therm, attached. 



Observation at bottom 29*400 - 50^ 

 Observation at top - 25*190 - AG 



Difference 4 



Expansion for 1® at 25 inches - '0025 

 Multiply by difference - - 4 



•0100 



One hundredth of an inch is therefore to be added to the 

 observed height of the upper barometer 25*190, so the cor- 

 rected height is 25*200. It is, however_, to be observed, that 

 the application of this correction is of doubtful accuracv in 

 practice; as it is by no means certain that the attached ther- 

 mometer, be it placed where it may in the mounting, will 

 give the real heat of the column of mercury in the barome- 

 ter, and therefore I had at first paid that it niight on the 

 whole, in general practice, be neglected. If much accuracy 

 •is wished, and time permits, the surest way is*to leave the 

 barometer in the shade so long as for the whole inslrumeut 



to 



