410 



Biographical Account of 



[JUNB, 



methods proposed to obviate them. It was upon this occasion 

 that the English, who had hitherto dene nothing respecting the 

 grand geographical operations in which the French had distin- 

 guished themselves, signalised themselves in their turn by 

 methods which surpassed every thing that had been hitherto 

 done. It was then likewise that MM. Cassini and Legendre 

 made the first trial of the circle of Borda. 



Bouguer, at the end of his measure of a degree in Peru had 

 endeavoured to determine the attraction of mountains, and the 

 quantity which they draw the plumb-line of the sector from the 

 meridian. He had found a real and indisputable attraction; but 

 one-half less than ought to have resulted, from the size of the 

 mountain. Hence he concluded that it was hollow within, and 

 undermined by a volcano. Duul^ts might be entertained of a 

 result obtained by means of instruments of middling goodness, 

 Bouguer had himself expressed a wish that the experiment were 

 undertaken in Europe with more care and with better instru- 

 ments. Dr. Maskelyne undertook this inquiry, with the sector 

 that he had with him at St. Helena, after having corrected the 

 suspension, and altered the division. He made choice of Sche- 

 hallien, a mountain in Scotland. It will be necessary to consult 

 his memoir, in order to see the care and the pains which this 

 operation cost him which appears so easy. He found b'S'^ for 

 the derangement of the thread by the attraction of the mountain; 

 he concluded from it that the density of the mountain ought to 

 he one-half of the mean density of the earth. It results from 

 this, that the density of the interior of the earth is greater than 

 that of its surface. This had been already proved by the mea- 

 surement of degrees, and by the pendulum. Finally, he con- 

 cluded that the densisty of the earth p four or five times greater 

 than that of water. Cavendish, by experiments of another 

 nature, found afterwards five and a half for the density of the 

 earth. But he himself had some doubts about the extreme 

 precision of liis result, and as that of Maskelyne is likewise 

 founded upon suppositions not rigorously exact, we may, till 

 new espriments be made, suppose the density of the earth to be 

 five times that of water. Finally, Dr. Maskelyne admits it as 

 veiy possible that the unequal density, even at the surface, may 

 have occasioned the differences observed in the measurement of 

 different degrees. 



Such are the principal memoirs published by Dr. Maskelyne, 

 but he left a great many others in manuscript, and philosophers 

 will doubtless learn with pleasure that the care of publishing 

 them has been entrusted to Mr. Vince, Professor of Astronomy 

 and Experimental Philosophy at Cambridge, known by a treatise 

 on astronomy, and by the description of the most modern instru- 

 ments. We shall find perhaps some nevv details on a micrometer 



