Report on the Progress and Stale of Applied Mechanics. 225 



practice in the French navy, the ships of which became examples in 

 tliis respect to the rest of the world. The building of ships of the 

 forms most suitable for speed, was at fii'st cultivated chiefly by the 

 Amei'icans, who attained great perfection in it by practical experience. 

 Of late years it has also been cultivated by ourselves with good success, 

 and in a great measure by practical trial alone. The Wave-line 

 System of Mr. Scott Russell was deduced by him from experiments on 

 the resistance of boats and transmission of waves, and has been put 

 in practice with success, both by Mr. Russell himself, and by other 

 builders. It must be admitted that the main principles of that system, — 

 viz., that the figure of the water-lines of a ship should be that of a wave, 

 and that the lengths of her bow and stern, best suited to a given speed, 

 are respectively the half-lengths of waves of certain kinds which travel 

 natm-ally with that speed, — are founded to some extent on analogy, and 

 not on strict demonstration. But even supposing that Mr. Russell's 

 system is not absolutely the best, it is certain that it must be a good 

 approximation towards the best system. 



69. Considerable progress has of late been made both in the theoreti- 

 cal determination and in the practical use of the law which connects 

 the FIGURE, SIZE, AND SPEED OE A SHIP, with the POWEE required for 

 her PEOPTJLSION. Experiments on models have thrown but little light 

 on this problem, which can be solved only by accurate recording of the 

 results of experience on the lai'ge scale. 



70. The consideration of ships, which are machines as well as struc- 

 tures, has led us to the subject of machines m general. Machines 

 may be considered in three lights : — First, with respect to the prime 

 mover which drives them ; secondly, with respect to the transmission 

 of power and motion from the prime mover to the working parts ; 

 thirdly, with respect to the nature of the useful work performed. 



71. The Eeficienct of a machine is its economy of power or energy ; 

 that is, the ratio of the useful work performed to the energy exerted. 

 Work is measured or expressed by the product of a resistance into the 

 distance through which it is overcome ; for example, by the product of 

 so many pounds into so many feet, called so many /oot-2)owuls. Enerijy, 

 or the capacity for performing work, is expressed in the same way. In 

 all machines, — indeed, in all systems of bodies acting on each other in 

 any way, and in the universe itself, the Energy exerted is equal to the 

 work performed : but in machines, constructed by human art for human 

 purposes, part of the work always consists in overcoming resistances 

 foreign to the purposes of the machine, and is said to be lost or vmsteiJ. 

 The remainder is the Uaefvl Work., which, together with the lost work, 

 makes a total of work equal to the energy expended. The great end of 



Vol. IV.— No. 1. 2 a 



