WILLIAM SIEMENS, 



359 



flea-shore, is influenced by the rising land of continents, or by 



cavities in the interior of the earth. He also established a reason 



li>r an observation made previously by Airy, that total gravitation 



;iter oil an island than it is near the sea-shore of a continent, 



i eater on the sea-shore than on an estuary inland.* 



KMI-LOV.MK.NT OF SECONDS PENDULUM. The seconds pendulum 

 has been the instrument employed in all cases to determine varia- 

 tions in the total attraction of the earth upon its surface, this 

 being the method first proposed and adopted by Newton. 



SPIRAL SI>KI\<; PROPOSED BY HERSCHEL. Sir John Herschel 

 has proposed to use instead of the pendulum a weight attached to 

 a spiral spring, and he has shown that with increase of the force 

 of gravitation, the spring must be proportionately elongated. Sir 

 John Herschel writes, that " the great advantages which such an 

 apparatus and mode of observation would possess, in point of con- 

 venience, cheapness, portability, and expedition, over the present 

 laborious, tedious, and expensive process, render the attempt to 

 perfect such an instrument well worth making." f It appears, 

 however, that this proposal by Sir John Herschel has never been 

 practically realized, and that, indeed, no serious attempt has been 

 made to construct an instrument of such delicacy as to show 

 statically minute variations in total gravitation, notwithstanding 

 the great oscillations to which a weight so suspended would be 

 liable, and notwithstanding the influence of changes of tempera- 

 ture and atmospheric density. 



(iKNERAL CONDITIONS. Neither the pendulum nor the appara- 

 tus suggested by Sir John Herschel would be applicable to the 

 measurement of the height of a mountain or plateau above the 

 sea-level, owing to the considerable error which would be caused 

 by changes in gravitation, through the local attraction of the mass 

 of the mountain itself above the horizon, nor would either instru- 

 ment be serviceable on board ship for obvious reasons. But if an 

 instrument could be devised which would be capable of indicating 

 extremely slight variations in the total gravitation of the earth, 

 subject only to comparatively slight causes of error, it would be 

 found, I contend, that these indications would vary with the 

 varying depth of water below the instrument, in such a definite 



* Cambridge's Philosophical Transactions, Vol. VIII. pp. C/2-695. 

 t Herschel's " Astronomy," Cabinet Cyclopaedia, foot-note, p. 125. 



