If the Earth is a globe, there cannot be a question that, however irregular the _land_ may be in form, the _water_ must have a convex surface. And as the difference between the true and apparent level, or the degree of curvature would be 8 inches in one mile, and in every succeeding mile 8 inches multiplied by the square of the distance, there can be no difficulty in detecting either its actual existence or proportion. Experiments made upon the sea have been objected to on account of its constantly-changing altitude; and the existence of banks and channels which produce a “a crowding” of the waters, currents, and other irregularities. Standing water has therefore been selected, and many important experiments have been made, the most simple of which is the following:--In the county of Cambridge there is an artificial river or canal, called the “Old Bedford.” It is upwards of twenty miles long, and passes in a straight line through that part of the fens called the “Bedford level” The water is nearly stationery--often entirely so, and throughout its entire length has no interruption from locks or water-gates; so that it is in every respect well adapted for ascertaining whether any and what amount of convexity really exists. A boat with a flag standing three feet above the water, was directed to sail from a place called “Welney Bridge,” to another place called “Welche’s Dam.” These two points are six statute miles apart. The observer, with a good telescope, was seated in the water as a bather (it being the summer season), with the eye not exceeding eight inches above the surface. The flag and the boat down to the water’s edge were clearly _visible throughout the whole distance!_ From this observation it was concluded that the water did not decline to any degree from the line of sight; whereas the water would be 6 feet higher in the centre of the arc of 6 miles extent than at the two places Welney Bridge and Welche’s Dam; but as the eye of the observer was only eight inches above the water, the highest point of the surface would be at one mile from the place of observation; below which point the surface of the water at the end of the remaining five miles would be 16 feet 8 inches (5² × 8 = 200 inches). This will be rendered clear by the following diagram:-- [Illustration: FIG. 2.] Let A B represent the arc of water from Welney Bridge to Welche’s Dam, six miles in length; and A L the line of sight, which is now a tangent to the arc A B; the point of contact, T, is 1 mile from the eye of the observer at A; and from T to the boat at B is 5 miles; the square of 5 miles multiplied by 8 inches is 200 inches, or, in other words, that the boat at B would have been 200 inches or above 16 feet below the surface of the water at T; and the flag on the boat, which was 3 feet high, would have been 13 feet below the line-of-sight, A T L!!