THE GROWTH OF THE STEAM-ENGINE. 537 



this loss by keeping up the temperature of the interior of the cylinder, 

 and thus preventing, in some degree, this deposition, and by reevap- 

 orating this moisture during expansion, and thus deriving useful effect 

 from heat so expended before the exhaust-valve opens, and it is 

 thx*own unutilized into the condenser. 



James Watt, therefore, applied the steam-jacket more wisely than 

 he knew, for this matter was not, in his time, understood. Indeed, he 

 gave up its use, thinking it could have no possible economical value, 

 but the consequent falling off in the duty of the engine induced him 

 to restore it, and we still find it on the Cornish engine of to-day. 



103. This loss is also, in some degree, prevented, by dividing the 

 expansive working of the steam among two or more cylinders, as in 

 the compound or Woolf system described in the preceding lectures. 



Here the heat wasted in either cylinder is less, in consequence of 

 the lessened range of temperature, and that lost by one cylinder is 

 carried into the second, and then, to some extent, utilized. 



The amount of saving effected by these means is quite consider- 

 able so great, in fact, as to have produced a complete revolution in 

 engineering practice in the construction of marine engines by the 

 best-known builders. 



They have, under the lead of John Elder, adopted the Woolf 

 engine, which had, in earlier times, with lower steam, less expansion, 

 and less intelligent engineering, proved apparently a failure. 



104. To-day, nearly all sea-going steamers are fitted with such 



