PACIFIC EXPLORATION: L. J. BRIGGS 
405 
the effect of vertical motion. The expansion of the top of the column into a 
bulb having a cross-sectional area 500-800 times that of the capillary reduces 
proportionally the change in level in comparison with the actual movement 
of mercury in the capillary. The rate of change in the acceleration of gravity 
with latitude is so slight that extensive damping is permissible from this 
standpoint, although the time required to secure an observation is of course 
correspondingly increased. 
Correction for the course and speed of the ship. — Eotvos^^ has shown 
the necessity of applying a correction for the easterly or westerly motion 
of the ship, due to the fact that the ship's motion modifies the angular 
velocity of revolution of the apparatus about the earth's axis. The 
centrifugal force acting on the mercurial column when on board a ship 
moving east or west is therefore not the same as when the ship is at 
rest or moving north or south. The correction may be as great as 1 
part in 10,000, but can be accurately computed if the course, speed, and 
approximate latitude of the ship are known. 
The probable error of the observations. — In 1914, observations were 
made from Sydney, Australia, to San Francisco by way of Wellington, 
N. Z.; and in 1915, two instruments were taken from New York to 
San Francisco via Panama. The following table shows the results of 
gravity determinations on board ship in various harbors during the 
two voyages, and where pendulum observations are available near the 
stations they are appended for comparison. The 1915 observations 
show a mean probable error in the harbor determinations of 13 parts 
in 1,000,000. The readings of the two instruments at sea were for the 
most part as consistent as at the harbor stations. The ocean measure- 
ments will be discussed in a later paper. 
The method is by no means to be considered as perfected. One of 
the instruments used in 1915 gave results consistently lower than the 
other. This indicates a systematic error which must be located. It is 
also highly desirable that new instruments constructed of boro-silicate 
glass should be carried over the same course several times with different 
sea conditions in order to determine whether systematic errors are 
introduced into measurements made on a rough sea. 
Acknowledgments. — In the first apparatus constructed, the gas 
chamber and capillary were made of steel. This apparatus developed 
a leak on the voyage to Sydney, and repeated attempts to repair it in the 
Fiji Islands and at Sydney met with failure. This disappointment 
was however more than offset by the kindness of Dr. J. A. Pollock, 
Professor of Physics in the University of Sydney, who placed the facili- 
ties of his laboratory and the services of his glass-blower and mechanician 
