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this loss by keeping up the temperature of the interior of the cylinder, and thus preventing, in some degree, this deposition, and by reëvaporating this moisture during expansion, and thus deriving useful effect from heat so expended before the exhaust-valve opens, and it is thrown 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 considerable—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.

PSM V12 D555 John Elder.jpg
John Elder

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