Plumb-line in the Hawaiian Islands. 315 



least several seconds, for a mountain equal to the one under 

 consideration. 



However, when the difference between the astronomical and 

 geodetic latitudes is nearly a minute of arc, this method will 

 give a value for the ratio of the density of the mountain, to 

 that of the earth, in which the error is not greater than one- 

 tenth of the whole amount. The difference between Play- 

 fair's reduction of the observations on Schiehallien and the 

 generally accepted value is about one-fifth of the whole. 



The last eruptions from Haleakala took place at least many 

 hundred years ago. The natives have no traditions bearing 

 on the fact ; and geologists believe that the immense crater has 

 come to its present condition by the sinking in of the top of 

 the mountain and not by erosion. Both these would indicate 

 a density of the mountain at least equal to its surface density : 

 probably more. The pendulum adds testimony in this direc- 

 tion, and the zenith telescope observations at the foot of the 

 mountain, indicate the same thing. 



Therefore we may assert that the defect of gravity, usually 

 noted on continental mountains, does not apply to those on 

 small islands in the middle of a deep sea. Whether this sup- 

 posed defect is caused by assuming an incorrect mean sea level, 

 or whether the mountains are indeed only vertically and not 

 horizontally displaced matter, we cannot now state. Both 

 causes may exist : one certainly does. 



On the Island of Hawaii there appears to be a striking ex- 

 ample of plumb-line deflections. At Kohala we have a deflec- 

 tion of half a minute towards the south, and at Hilo, one, of 

 quarter of a minute towards the north. These points are both 

 on the windward coast and are distant from each other about 

 sixty miles. 



It is easy to see why there should be one to the southward, 

 at Kohala, for the great mountain masses of Mauna Kea, 

 Mauna Loa and Hualalai would all have this influence. But 

 at Hilo there is no explanation unless we assume that the south 

 side of the island, where the volcanoes are active, is much less 

 dense than the north side, where the fires have been slumber- 

 ing for centuries. 



In general the deflections everywhere in the islands are 

 greater than we expect, judging from analogy with conti- 

 nental masses : but here we have a surrounding sea which is 

 much lighter than the land on which the continental moun- 

 tains rest ; and it is a question whether the sea bottom is really 

 very much more dense than the average land. If the earth in 

 solidifying has simply contracted unequally, and if each prism, 

 having its base on the surface and its apex at the center, con- 

 tains the same amount of matter, this would not make the bed 



