426 



TABLES 



FOR REDUCING BAROMETRICAL OBSERVATIONS TO THE LEVEL. OF THE SEA, OR TO 



ANOTHER LEVEL. 



To reduce barometric means taken at a given elevation to the heiglit they would 

 have if taken at the level of the sea, or barometric observations made at different 

 elevations to a common level, in order to eliminate the influence of altitude in the 

 comparison of barometric pressures, is a problem, the solution of which is often 

 needed in meteorology. 



For a complete and accurate reduction, embracing all cases, Tables IV. and V., 

 by Dippe, given above, pages 54 et seq., may be used. But when the difference of 

 height between the two stations, or above the sea-level, does not exceed a few 

 hundred feet, the small tables XVI. to XIX., in three ditTerent scales, will be found 

 more convenient. 



Ta'bles XV[. and XVll. have been computed from the constants of Laplace\s for- 

 mula, the barometric coefficient, including the correction for the decrease of gravity 

 on a vertical, being respectively 60,345.51 English feet and 56,6*21.83 Paris feet; 

 and the coefficient for expansion of moist air 0.00222 and 0.005, 



In Table XVlll. the coefficient 18,420 metres, deduced from Regnault's experi- 

 ments (see Proceedings of Llie Ainer. Assoc, for Adv. of Science, 1857), and his co- 

 efficient for expansion of dry air, 0.003665, increased to 0.0039, in order to include 

 the effect of moisture, have been used. 



Use of the Tables. 



The correction for reducing the barometer to the level of the sea is found by the 

 formula 



where C is the correction required ; f, the elevation of the station ; N, the number 

 in the tables ; A', the reading of the barometer ; h, the normal height of barometer 

 at the sea-level. 



Exa7nple. 



At Cambridge Observatory, Massachusetts, at 71.34 English feet above mean tide, 

 the mean barometer is = 29.939 inches ; the mean temperature 47°. 3 Fahrenheit ; 

 what would be the height at the level of the sea.' 



In Table XVI. we take for 47°. 3 = 90.49, or, in order to get the correction in a 



fraction of an inch, 904.9. 



Then 



„ 71.34 29.939 . _. . . , 



L == X — qn^ ^= 0.079, correction requu-ed ; 



and 

 29.939 + 0.079 =: 30.018 inches, height of the barometer at the level of the sea. 



It will be seen that the quantity represented by the second member can be neg- 

 lected without causing a sensible error in the correction. In this case the error does 

 not amount to .001 ; it scarcely would reach .002 for 250 feet of elevation ; so that 



the reduction can be made in most cases by a simple division ; viz. ^. 

 D 86 



