BAROMETRICAL TABLES. XXXV 



height of the mercurial column must not only be reduced to what its height 

 would be if at a standard temperature, but also to what it would be at a 

 standard value of gravity. 



As stated on page xviii, the standard value of gravity adopted is 980.665 

 dynes. At the time of its adoption this value was assumed to apply for 

 "latitude 45° and sea-level" on the basis of the absolute determination of 

 g at the International Bureau by Defforges, 1 887-1 890 (Proces-Verbaux, 

 Comite Inter, d. Poids et Mesures, 1887, pp. 27-28, 86; 1891, p. 135). 



More recent determinations, l based upon numerous measurements 

 in all parts of the world, and assuming a certain ideal figure for the earth, 

 give for the mean value of g at latitude 45 and sea level the value 980.621 

 dynes. This differs from the standard value by 0.044 dyne. Departures 

 of this magnitude from the mean sea-level gravity of a given latitude are 

 frequently encountered, and in some cases surpassed. They are attributed 

 to topography and isostatic compensation, and to gravity anomalies. For 

 example, according to Bowie, 2 at Pikes Peak, Colo., the correction for 

 topography and compensation is +0.187 dyne, while the gravity anomaly 3 

 is 4- 0.021 dyne, giving a total gravity departure of +0.208 dyne. Also, 

 at Seattle, Wash., from the mean of measurements at two stations, the cor- 

 rection for topography and compensation is — 0.019 dyne i and the grav- 

 ity anomaly is — 0.093 dyne, 5 giving a total gravity departure of — 0.112 

 dyne. The gravity departure at Pikes Peak is sufficient to cause the barom- 

 eter to read 0.004 inch or 0.10 mm. low, while the departure at Seattle is 

 sufficient to cause the barometer to read 0.003 inch or o.O() mm. high, as 

 compared with what the readings would have been with gravity at normal 

 intensity for the latitudes of the respective stations. 



From the foregoing it is evident that the value of local gravity, gi, at 

 the observing station must be determined before the barometer reading 

 can be accurately reduced to standard gravity. In many cases, and espe- 

 cially at sea, it is not practicable to measure g t . In the United States its 

 value may frequently be determined with sufficient accuracy in the follow- 

 ing manner: 



(1) Compute g^, mean gravity at sea level for the latitude of the sta- 

 tion, from the equation 6 



U = 97 8 -039 (l + 0.005294 sin 2 — 0.000007 sin 2 20), 

 = 980.621 (1 — 0.002640 cos 20 + 0.000007 cos 2 20) 



(2) Correct g$ for altitude by the equation 7 

 c (dynes) = — 0.0003086 h (meters), or 

 c (dynes) = — 0.000094 ^ (feet), 



1 Investigations of gravity and isostasy, by William Bowie. U.S. Coast and Geodetic 

 Survey, Special Publication No. 40, 1917, p. 134. 



2 Op. cit. p. 50. 3 Op. cit. p. 59. * Op. cit. p. 50. » Op. cit. p. 59. 

 6 Bowie, op. cit. p. 134. 7 Bowie, op. cit. p. 93. 



