THE 



AMERICAN JOURNAL OF SCIENCE 



[FOURTH SERIES.] 



Art. I. — On Gravity 



Determinations at 

 Battek. 



Sea;* by L. A. 



A given mass changes its weight when transported over the 

 Earth's surface. How shall we determine these alterations? 

 It cannot he done with a balance ; for a mass in one scale pan 

 held in equilibrium by a set of weights in the other pan will 

 remain in equilibrium all over the Earth, since the Earth's 

 gravitational force within our limits of measurement acts alike 

 on all substances. The variations in gravity are composed of 

 three parts: one called the "normal part," varying simply with 

 latitude; another the "anomaly," a more or less irregularly 

 distributed part, and the third part dependent upon altitude 

 of station above sea-level. 



"We might replace the beam of our balance by a pivoted 

 magnetized steel bar or needle. Suppose we were to start out 

 from Washington, where the magnetic dip is about 70*5°, with 

 the beam made to lie horizontal by suspending a suitable 

 weight on its south end; we have thus balanced the earth's 

 gravitational force by the vertical component of the Earth 's 

 magnetic force. Were we now to proceed with this balance 

 to some distant point, the beam might no longer be truly hori- 

 zontal but make instead an angle with the horizon, the magni- 

 tude of which would depend upon the relation at that point 

 between the earth's magnetic and its gravitational force. 

 Knowing the value of the vertical magnetic component, it is 

 then possible to determine, within certain limits, from the 

 inclination of the beam how much the suspended mass has 



* Presented before the Philosophical Society of Washington, November 5, 

 1910. 



Am. Jour. Sci. — Fourth Series, Vol. XXXI, No. 181. — January, 1911. 



