366 



MY LIFE 



[Chap. 



line for the six miles, then the three objects — the telescope, 

 the top disc, and the black band — being all exactly the same 

 height above the water, the disc would be seen in the telescope 

 projected upon the black band ; whereas, if the six-mile 

 surface of the water is convexly curved, then the top disc 

 would appear to be decidedly higher than the black band, the 

 amount due to the known size of the earth being five feet 

 eight inches, which amount will be reduced a little by refraction 

 to perhaps about five feet. 



The above diagrams illustrate the experiment made. The 

 curved line in Fig. i, and the straight line in Fig. 2, show the 

 surface of the canal on the two theories of a round or a flat 

 earth. A and C are the two bridges six miles apart, while B 

 is the pole midway with two discs on it, the upper disc, the 

 telescope at A, and the black line on the bridge at C, being 

 all exactly the same height above the water. If the surface 

 of the water is truly flat, then on looking at the mark C with 

 the telescope A, the top disc B will cover that mark. But if 

 the surface of the water is curved, then the upper disc will 

 appear above the black mark, and if the disc is more than 

 four feet above the line joining the telescope and the black 

 mark, then the lower disc will also appear above the black 

 mark. Before the experiment was made a diagram similar 

 to this was submitted to Mr. Hampden, his referee Mr. 

 Carpenter, and Mr. Walsh, and all three agreed that it 

 showed clearly what should be seen in the two cases, while 



