240 LECTURE XXVI. 



required for each flood gate may be determined in the same manner as the 

 thickness of a sluice. 



Where a sluice board of considerable dimensions is to be occasionally 

 raised, it may be necessary to ascertain the force which will be required for 

 overcoming its friction ; this friction is nearly proportional to the whole 

 pressure of the water, and may be found, with sufficient accuracy, in pounds, 

 by multiplying the square of the depth of the sluice, in feet, by 10. Thus, 

 if the depth be 3 feet, the friction or adhesion will be about 90 pounds for 

 each foot of the breadth. 



If the side of a canal gives way, it is sometimes of consequence to pre- 

 vent, as much as possible, the escape of the water. For this purpose it is 

 usual to have doors or valves in various parts of the canal, which, when the 

 water is at rest, lie nearly flat at the bottom ; but when it begins to run 

 over them, with a considerable velocity, they are raised by its force, and 

 put a stop to its motion. 



The utility of the introduction of canals into a commercial country may 

 be estimated in some measure by the effect of the same labour, employed in 

 removing weights by land carriage and by water. Thus, a single horse can 

 scarcely draw more than a ton weight on the best road, but on a canal, the 

 same horse can draw a boat of 30 tons at the same rate. 



The construction of piers and quays, and the management of harbours, 

 are also important departments of hydraulic architecture ; it often happens 

 that besides the application of the general principles of mechanics and 

 hydrostatics to these purposes, the peculiar circumstances of the case may 

 indicate to an ingenious artist a mode of performing the required work in 

 an effectual and economical manner. We may find a good example of such 

 an arrangement, in the account given, by Mr. Smeaton, of the method 

 which he adopted for the improvement of the port of Ramsgate,* and which 

 indeed resembles some that had been before employed in similar cases : by 

 forming a large excavation, which is furnished with flood gates, and is con- 

 stantly filled at high water, he has procured a number of artificial torrents, 

 which escape through the sluices, and become powerful agents for carrying 

 away the matter deposited by the sea, and tending to impede the navigation 

 of the harbour. 



LECT. XXVL ADDITIONAL AUTHORITIES. (See LECT. XXIV.) 



Specific Gravities. Marinus Ghetaldus, Promotus Archimedes, 4to, Romse, 

 1603. Boyle's Works, 1772. Tables of Specific Gravities, Ph. Tr. xv. 927; 

 xvii. 694 ; xxvii. 206, 511 ; xxxiii. 114 ; xlv. 416, the last by Davis. Brisson, Pe- 

 santeur Spec, des Corps, 4to, Paris, 1787. Ramsden on the Sp. Gr. of Fluids, 4to, 

 1792. Atkins on Sp. Gr. 4to, 1803. 



Hydrometers. Boyle's, Ph. Tr. 1675, p. 329. Moncorie's, Birch, i. 257. Horn- 

 berg's Areometer, Hist, et Mem. 1699, p. 46. Irwin, Ph. Tr. 1721, p. 223. 

 Fahrenheit, Areometri Descriptio, ibid. 1724, p. 140. Desaguliers on Clarke's 

 Hydrometer, ib. 1730, p. 277. Gesner de Hydroscopico, Zurich, 1754. On 

 Areometers, Hist, et Mem. 1768, p. 435 ; 1770, p. 526 ; Ph. Tr. 1778, p. 509 ; 

 1788, p. 582 ; 1793, p. 145. Roz. Jour, xxxiii. 241. Mem. della Soc. Ital. vii. 79. 

 Annales de Chimie, xxi. 3 (Guytoris), xxvi. 3, 132 ; xxviii. 3, 282 ; xxxi. 12^ ; 



* Smeaton on Ramsgate Harbour, Lond. 1791. 



