Military/ Bridges, and the Passage of Rivers. 291 



which, by this rule, the mean will be readily obtained ; and it is 

 the mean velocity that must be used in calculating the discharge, 

 supply, force, and every other effect of running water. 



Several experiments are resorted to for the purpose of deter- 

 mining the mean velocity of running water, such as a cylindrical 

 rod of wood, loaded at the lower extremity with a piece of lead, or 

 straight glass tubes, having their bottoms filled with small shot, 

 to keep them perpendicularly in the water, and at such a depth 

 as may suit the depth of the stream. In absence of these, 

 weeds or shrubs, having a proper quantity of earth adhering to 

 their roots, may be conveniently substituted, when great preci- 

 sion is not required. By using a sufficient quantity of these at 

 different points in the breadth of a stream, the mean velocity of 

 the whole flowing mass of water may be obtained with consider- 

 able accuracy. By observing the mean time in seconds by a 

 good watch, or half or quarter seconds pendulum, the volume 

 of water that flows through a given space or distance in a given 

 lime, may be accurately computed, which may be turned to 

 considerable advantage in many philosophical inquiries*. 



From a vast^ mass of experiments, Du Buat proceeds to de- 

 duce an empirical formula, on the principles that have now been 

 detailed, and at last arrives at the following expression for the 

 mean velocity of' running water^ in French inches, per second of 

 time. 



In which V denotes the mean velocity of the water in French 

 inches, r the radius of the section, and b the distance, divided 

 by the fall in the same measure, that is, if the fall be two feet 



2 11 



in a mile, then -^^ = ^gj^ = t, and therefore h = 2640. 



By r, or the radius of the section, is meant the area of the 

 transverse section of the river in square inches, divided by the 

 linear surface of the water in contact with its banks. Or it may 



• In Dr Thomas Thomson's Treatise on Heat and Electricity, page 268, 

 a comparison is made between the fall of rain, and quantity of evaporation, by 

 means of the discharge on the Clyde. Though the principle is good, yet we 

 doubt the accuracy of his result. 



T 2 



