Figure by means of the Pendulum. 453 



precision the disturbing effect of this huge mass of lava on a 

 plumb line suspended north or south of it. But the mass of 

 the mountain enters as one of the unknown quantities. This 

 quantity was determined by measuring the force of gravity at 

 the sea-level and at the highest practicable point by means of 

 the pendulum. Knowing the mass of the mountain and there- 

 fore its mean density, its attraction at any given point is easily 

 deduced. JSTow the result of the work was this : the pen- 

 dulum observations showed that the mean density of the 

 mountain was very nearly one-half that of the earth's mean 

 density, that is, that the island is a little more than two and 

 one half times as heavy as water. This value would lead us to 

 expect, at the point selected on the south shore, a disturbing 

 effect on our star observations of 28". When the two points 

 on the north and south sides of the island were connected by 

 triangulation a discrepancy of 29" was brought out. The 

 agreement between the results obtained by two independent 

 methods is so close as to give us considerable confidence in the 

 astronomical and geodetic parts of the work as well as in the 

 measurement of the force of gravity at the upper and lower 

 station. Besides this, rock specimens were secured from many 

 parts of the island at different elevations. Their densities 

 were carefully determined at the bureau of weights and 

 measures in the Coast Survey Office. When a mean value 

 was taken we arrived at the result that the mean density of the 

 mountain is somewhat more than that of the rocks found on 

 its surface. This is contrary to the result generally obtained 

 on mountains and high table lands ; and it is notably in oppo- 

 sition to determinations on continental mountains. But let us 

 remember in this connection that the sea level in the neigh- 

 borhood of continents may be considerably disturbed by the 

 attraction of the land, and that a single mountain in the middle 

 of a deep sea would have practically no influence in elevating 

 the surface of the ocean. In point of fact the island of Maui 

 could not elevate the surface of sea around it by more than 

 ten feet — a quantity easily neglected in this investigation. 

 One word about the correction for 



Continental attraction. 



We know that on a sphere at rest attraction. varies inversely 

 as the square of the distance from the center, but in the case 

 of a rotating spheroid this assertion is not true. The actual 

 diminution of gravity from the pole to the equator is about 

 2-i-o-th part of itself — this is in part due to the centrifugal 

 force in consequence of the earth's rotation and in part to the 

 spheroidal shape into which the earth has been thrown by this 

 rotation. Bououer was the first to call attention to the fact 



