298 



On Barometric Heights. By Captain R. Shortrkde, First Assistant 

 Grand Trigonometrical Survey. 



If I remember rightly, your correspondent D has given a formula 

 for computing Barometric heights, which to me appears to be neither 

 so simple nor easy of recollection as that given by Professor Leslie, 

 at the end of his Geometry ; which is " As the sum of the mercurial 

 columns is to their difference, so is the constant number 52,000 to 

 the approximate height" in feet. This rule is easily remembered, 

 and is not far from the truth ; but a more correct result may be ob- 

 tained by using 52,200 as the 3rd term. At the height of a mile 

 the height thus found differs only nine feet in defect from that obtain- 

 ed by a logarithmic calculation, whereas by Leslie's rule the defect 

 is twenty-nine feet. When the height does not exceed 4,000 feet, 

 52,200 gives within two feet of the logarithmic calculation. At ele- 

 vations above a mile, the difference increases rapidly : it then becomes 

 necessary, as Leslie recommends, to subdivide the interval into smaller 

 portions. 



The following Table shews the results of the several Rules. 



At 3700 feet by using 



52,200 we get exactly the 



same result as by logarithms. 



Leslie's rule is then in 



defect about 15 feet. 



This rule may be thus 

 expressed in words : " The 

 sum of the barometric co- 

 lumns at the two stations 

 is to their difference, as 

 52,200 to the approximate 

 height in feet," and alge- 

 braically (B and b being the 

 barometers at the 2 stations) 





Approximate 



Eieigbtby 





Barometers. 



Logarith- 

 ms. 



52,200. 



52,000. 



30 and 29.5 



438.0 



438.7 



437.0 



••• 5 



, 29.0 



883.4 



884.7 



881.3 



... , 



, 28.5 



3336.6 



1338.0 



1333.3 



... , 



, 28.0 



1798.0 



1800.0 



1793.1 



... , 



, 27.5 



2267.3 



2269.1 



2260.4 



... , 



, 27.0 



2745.4 



2747.4 



2736.8 



... , 



, 26.5 



3232.5 



3233.6 



3221.2 



... , 



, 26.0 



3728.9 



3728.6 



3714.3 



... 



, 25.5 



4234.9 



4232.4 



4216.2 



... , 



, 25.0 



4750.9 



4745.5 



4727.3 



... , 



, 24.0 



5814.6 



5800.0 



5777.8 



... , 



, 23.0 



6923.6 



6894.3 



6867.9 



... , 



, 22.0 



808L9 



8030.8 



8000.0 



... , 



, 21.0 



9294.1 



9211.8 



9176.5 



... , 



, 20.0 



10565.5 



10440.0 



10400.0 



(l+D 52200= Approximate height (A). 



