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for ascertaining the dimensions and the figure of the earth ? For 

 this purpose we must go to astronomy. One thing which must be 

 fully understood by you is this, that when we speak of observing a 

 star, we are observing something so prodigiously distant that the 

 direction in which we look at it may be taken as absolutely the 

 same from whatever part of the earth we view it. The nearest star 

 of which we have any accurate notion of distance is distant more 

 than two billions of miles. I have here, a map of a portion of the 

 earth with its proper curvature, and I am sorry since I made it 

 that I did not give it an improper curvature and make it more 

 convex, as it would have been more intelligible. It contains the 

 British Islands, and it shows the degree of curvature the earth, has 

 in that extent. This (showing an apparatus upon the table, a rude 

 imitation of a Zenith Sector,) is a very rough affair. It represents 

 an astronomical zenith sector, or an instrument for measuring the 

 apparent distances of stars from the point over-head. Starting at 

 the Isle of Wight, I direct that telescope to a certain star there. 

 Suppose now that I carry the same instrument to a place in 

 Shetland, and I look at the same star. In consequence of this 

 curvature of the earth's surface, my telescope points too high, and 

 I must give it a certain degree of inclination in order to catch the 

 same star. That change of inclination in passing from the Isle of 

 Wight to Shetland is not because the place of the star is moved, 

 but because the direction of the footing of the instrument has 

 changed. It does not stand on the same level here as it did there, 

 and that degree of tilt which I have been obliged to give to the 

 telescope in order that, having seen the star from that place with 

 the telescope in one position, I may see the star from another 

 place — that degree of tilt is the genuine degree of change in the 

 position of the ground. That gives me a measure of the curvature 

 of the earth. So that, by these two processes, one of triangulation 

 — measuring enormous distances with great accuracy; and the 

 other — by observing the same star with the same instrument at 

 different places, and so getting a distinct difference in the curvature 

 of the earth, — by these we can compute the diameter of the earth 

 supposing it to be a sphere. This is the manner in which all the 



