rORTABLE BAROBIETEBiL 



Carreotion for 

 the difference 

 *t" k rupciatui€ 

 above the 

 fceezing point, 



Very simple 



rule for this 

 purpose. 



EymnpTcs to 

 illHstrare the 

 computations, 

 and shvvi their 



se-rvations as relate to great elevations may be always recom- 

 puted by more rigorous methods at leisure. 



The correction of the heights thus obtained, for the tem- 

 , perature of the air above freezing, is, by Sir G. Shuckburg, 

 supposed to be ds the height of the thermometer, and to ha 

 ' 2,44 thousandths of the approximate height for each degree 

 of Fahrenheit ; additive when the temperature is above freez- 

 ing, and subtractive when below freezing. General Roy's 

 observations and experiments lead to a supposition that the 

 correction is not exactly as the height of the thermometer, 

 and that at about the temperature of 50° it amounts to 2.5 

 thousandths, and is less both much above and much below that 

 temperature. For the purpose of immediate computation, I 

 take the correction at 2,5 ; which, though certainly too great, 

 will in general be productive of very small error, and affords 

 a rule which is easily remembered^ and quickly applied. Tt 

 is this. For every four degrees tJiat the mean temperature of 

 the two detached thermometers exceeds 32°, add one hun- 

 dredth of the approximate height, as before obtained, to it; 

 for every 40*^ one-tenth ; and so for any greater or lesser num- 

 ber of degrees. I have not hitherto mentioned the correction 

 which, in fact, ought to be the first in order, viz. that, for the 

 difference of temperature of the two barometers themselves : 

 but this correction is, in general, so small as to be safely neg- 

 lected, and is besides easily to be made from Mr. Ramsden's 

 numbers, which are engraven on the scale of the attached 

 thermometer. It may not be improper to give an example or 

 two of the method already detailed. 



Observation at bottom, 

 Obsiervation at top, 



29.400 Therm, in air, 45 

 25.200 Therm, in air, 41 



2 I 54.600 

 ]Mcan, — I 27.300 



Mean heat, 

 Standard, 



86' 

 43 



32 



Difference, 1 1 



Dif5?rence, 



