2 L. A. Bauer — Gravity Determinations at Sea. 



altered its weight, or in turn find the change in gravitational 

 force. We have in fact such an instrument as here described, 

 viz., the dip circle used in determining the total magnetic force 

 by Lloyd 's method, one operation consisting of measuring the 

 angle of dip of a suspended magnetic needle having a load on 

 one end. Observations made thus on the "Galilee" in the 

 Pacific Ocean, covering a range in latitude from about 59° 

 North to 42 South, showed that the variation in weight of the 

 load over a large range in latitude might be taken into account. 

 With such a dip circle, the latitude variation of gravity could 

 just about be detected ; however, for smaller gravity variations 

 the instrument would not be sensitive enough, both because of 

 its construction and our inability to determine the magnetic 

 force with sufficient precision. 



The method most commonly in use on land for determining 

 the value of the linear acceleration of gravity, g, is by pendu- 

 lum observations. But this method has not been found feasi- 

 ble on board ship. If, then, it is desired to extend a gravity 

 survey so as to include the oceans as well as the land, other 

 experimental means than swinging pendulums must be devised. 

 The method thus far used to disclose gravity anomalies at sea 

 is that known as the "boiling point, mercurial "barometer 

 method." The principle here is to measure the counterbalanc- 

 ing effect of the elastic pressure of a vapor on a column of mer- 

 cury, once by reading the height of the mercurial column, the 

 counterbalancing vapor being the atmosphere ; next, by measur- 

 ing the atmospheric pressure prevailing at the time by deter- 

 mining the temperature of the boiling point of pure water. 

 Since the boiling point, other things being equal, depends 

 solely upon the atmospheric pressure, it will not vary as we 

 pass over the earth as long as the pressure is the same ; how- 

 ever, the height of the mercurial column, under the same con- 

 ditions, changes with variations in gravity. Hence the grav- 

 ity anomaly is found by a direct comparison of the atmospheric 

 pressure determined from the boiling point with that read off 

 on the mercurial barometer. Prior to the boiling-point method 

 for measuring the prevailing atmospheric pressure inde- 

 pendently of gravitational force, the use of the aneroid was 

 proposed ; however, the latter instrument is found too variable 

 and uncertain in its indications to possess the required sensi- 

 tiveness. 



Gruillaume in 1894 was led to suggest the nse of boiling 

 point thermometers in place of the aneroid, but Mohn* has 

 the credit of having made the first practical use of the method, 



* Mohn, H. : Das Hypsometer als Luftdrackinesser und seine Anwendung 

 zur Bestimmung der Schwerekorrektion. Christiania, Skr. Vid. Selsk. 

 Math.-naturw. Kl. I, 1899, No. 2 (1-69). 



