PACIFIC EXPLORATION: L. J. BRIGGS 
399 
A NEW METHOD OF MEASURING THE ACCELERATION OF 
GRAVITY AT SEA 
By Lyman J. Briggs 
BUREAU OF PLANT INDUSTRY, WASHINGTON, D. C. 
Read before the Academy. April 17, 1916. Received June 1 9, 1 9 1 6 
Introduction. — The desirability of more extensive and more accurate 
measurements of the acceleration of gravity at sea has recently been 
emphasized by Bauer^, Bowie^, and Hayford.^ Such measurements are 
essential in the accurate determination of the figure of the earth. Ocean 
gravity measurements now available are due almost wholly to Hecker,^ 
who employed the mercurial barometer-hypsometer method in an 
elaborate and extended series of measurements in the Atlantic, Pacific, 
and Indian oceans. In this method the outstanding difference at a 
given station between the atmospheric pressure as computed from the 
boiling point of water and directly observed with the mercurial barom- 
eter is attributed to the difference in the gravitational force acting on 
the mercurial column at the given station and at the standard station 
(Lat. 45°, sea level). 
There are certain difficulties inherent in the barometer-hypsometer 
method which greatly lessen its usefulness. (1) The atmospheric 
pressure must be determined in absolute measure by each method in 
order that the determinations may be comparable, so that systematic 
errors are serious; (2) the boiling point determinations must be carried 
out with the highest degree of refinement in order to secure even moder- 
ate precision in the determination of g. The observed boiling points 
are not simply differential measurements. The true temperature 
interval between the melting point of ice and the observed boiling point 
must be known in terms of the hydrogen-scale before reference can be 
made to vapor pressure tables for the determination of the atmospheric 
pressure. 
An independent measure of the accuracy attainable in the determi- 
nation of atmospheric pressure by the boiling point method is afforded 
by measurements of the 'fundamental interval' of standard mercurial 
thermometers. Waidner and Dickinson^ found in a study of the stand- 
ard mercurial thermometers of the Bureau of Standards that the funda- 
mental intervals varied through a range of 0.015C. during the ten-day 
period covered by the measurements. This variation they attribute 
in part to the sticking of the meniscus with resulting variation in the 
capillary pressure. 
The probable error of a fundamental interval determination in Waid- 
