48 



to that in Shetland, the ground is tilted say ten degrees, and the 

 distance is nearly seven hundred miles. If it is ten degrees for 

 seven hundred miles, we have only by a rule of proportion to find 

 out what the length would be for three hundred and sixty degrees. 

 That is one in thirty-six, and you have only to multiply seven 

 hundred by thirty six and you have the entire circumference of the 

 earth. That is the way the dimensions of the earth are ascertained. 

 Thus we come to the general conclusion that the earth is a sphere 

 eight thousand miles in diameter, and twenty-five thousand tniles 

 in circumference. 



When I speak of the surface of the earth, it must be under- 

 stood that we always mean the surface of the earth as referred to 

 the sea level, — to that level which, if the land were perforated with 

 canals, the sea would take there. Above it stand mountains, and 

 below it the depths of the sea. When you see a mountain, you 

 think it is a great thing, and that it should be taken into consider- 

 ation. Mountains are taken into consideration by persons going 

 accurately into the depths of science, — but they are very small in 

 proportion to the earth. Suppose we had a model of the earth 

 twenty-five feet in diameter, how much do you think the mountains 

 on such a sphere would rise above the level 1 One-fifth of an inch. 

 That would never be seen. It may be neglected entirely. The 

 greatest depression of the sea would be about one-fifth of an inch 

 also. Understanding that, we may say generally, that as far as 

 our accurate measures go, our earth is a sphere ; to which I shall 

 refer again, introducing a small modification of considerable 

 importance. 



Then, having got so far, there is another thing which is 

 important to our principal subject, and that is the density of the 

 matter of which the earth is formed. This is a subject which has 

 engaged the best experimentalists in a variety of ways. The first 

 of these experiments is a very celebrated one, known as the 

 Schiehallien experiment, so called as being made on the attraction 

 of the mountain SchiehaUien, in Perthshire, in the Scotch High- 

 lands. It ranges to the east and west, and is well adapted to the 

 measures of attraction, carried on in the north and south directions- 



