Ill On the Mean Density of the Earth. 



veral reports to the Royal Society, Mr. Smeaton announced 

 that he had discovered the mountain SchehaUien, one of 

 tlie Grampian hills in the north of Scotland, possessing the 

 desired properties in a very eminent degree; being a very 

 lofty and narrow ridge, very sleep, extending a great length 

 east and west, and very narrow from north to south. 



This hill was in conscrpience deemed sufficiently con» 

 venicnt for making the experiment; and a person, who 

 had heen an assistant to Dr. Maskelyne at the Royal Ob- 

 scrvaiory, was engaged by the Society, and sent down to 

 Scotland to take the necessary measures about the hill, to 

 ascertain its shape and magniuule by horizontal measure- 

 ments, and by vertical sections in a great many directions 

 and situations; and lastly, by placing a proper instrument 

 and plummet against the middle of the sides of the hill, to 

 observe, by zenith distances, the deviation of the plumb-line 

 towards the hill. Before the survey and observations were 

 quite completed, — at the request of the Society, Dr.Maske- 

 lyne himself went down to Scotland, to see how the busi- 

 iiess was carried on ; and brought back the account of the 

 survey, with the report that, having tried the plummet on 

 the opposite sides of ihe hill, each side attracted it between 

 5 and 6 seconds from the perpendicular, and, in fact, that 

 the sum of the two opposite attractions was just equal to 

 1 l-jfig- seconds. 



Thus, then, the original question was satisfactorily an- 

 swered in the affirmative, viz. that the hill, a mass of dense 

 rocks, did sensibly attract the plummet, and draw it aside 

 from the perpendicular direction of the earth's gravitation, 

 and that by a certain quantity. 



The next consideration was, whether and how these ob- 

 servations and measurements could be employed, in com- 

 parison with the magnitude and effects of the whole globe 

 of the earth, to determine its mean density, in comparison 

 with that of the mountain. This indeed was the grand 

 question, a point of the highest importance to natural phi- 

 losophy, of novel and of the most delicate and intricate 

 consideration, as well as a work oF immense labcur. Here 

 were to be calculated mathematically the exact magnitude 

 of the hill, its shape and form in every respect, the position 

 and situation of all its parts, the various elevations and de- 

 pressions, and the attraction on the plummets, by every 

 point and particle in the hill, as well as of the neighbouring 

 mountains on every side of it. Then there was to be cal- 

 culated, in like manner, the attraction of the whole magni- 

 tude and mass of the earth, on ihc same plummets. Lastly, 



liie 



