Then c X 1172—b=x-I-b+ a : 1172—b Therefore c X =x.
Thus, if the proposed temperature of the hot well be 100° (and it should not be higher to obtain a tole rable vacuum in the cylinder), and that of the injec tion be 50°, we have a=50°, 1172-100 b=loo°; hence 100— 50 —=21.44=x. That is, for every foot of the capacity of the stroke in the cylinder calculated as has been directed, or for every cubic inch of water evaporated from the boiler, about 21), cubic inches of water at 50° will be required to condense the steam.
But as the injection water may not be obtained so cold as 50°, and other circumstances may require an allowance, a wine pint of water for every inch boiled off, or for every cubic foot of the contents of the stroke in the cylinder, may be kept in mind as amply sufficient. This greatly exceeds the quantity necessary in a good Newcomen's engine, and by showing the more perfect condensation, points out the superiority of the new engine; for the Newcomen's engine, if working to the greatest advantage, should not be loaded to more than 7 pounds upon the inch, whereas Watt's engine bears a load not much less than 1 pounds, exclusive of friction, when making twelve 8-feet strokes per minute.
What has been now said is not a matter of mere cu riosity: it affords an exact rule for judging of the good working order of the engine. We can measure with accuracy the water admitted into the boiler during an hour without allowing its surface to rise or fall, and also the water employed for injection. If the last be above the proportion now given (adapted to the temperatures 50° and 100°), we arc certain that steam is wasted by leaks, or by condensation in some im proper place.
It is evident that it is of great importance to have the temperature of the hot well as low as possible, because there always remains steam in the cylinder of the same or rather higher temperature; possessing an elasticity which balances part of the pressure on the other side of the piston, and thus diminishes the power of the engine. This is clearly seen by the barometer
which Mr. Watt applies to his engines, and is a most useful addition to the proprietor. It shows him the state of the vacuum, and, with the height of the mer cury in the steam-guage, points out the real power of the engine.
Mr. Watt finds that, with the most judiciously constructed furnaces, it requires 8 feet of surface of the boiler to be exposed to the action of the fire and flame to boil off a cubic foot of water in an hour, and that a bushel of Newcastle coals so applied will boil off from 8 to 12 cubic feet, and that it requires about a cwt. of Wednesbury coals to do the same.
Having thus given a general view of the inventions of Mr. Watt, we shall proceed to give an account of the labours of his contemporaries and his successors. The variations, however, which the steam engine has undergone are so numerous, that it would be im possible in a work like this, intended for general readers, to embrace all those contrivances which have been published; so that we must confine ourselves to those which have been regarded as combining in genuity with practical utility.
Description of Mr. Hornblower's Engine with two Cylinders.
As this engine is now in more extensive use than any other variety excepting Mr. Watts, and not withstanding its similarity in principle to Mr. Watts, displays much ingenuity, it is necessary to give a full description of it. Ilornblower does not seem to have published any account of his own engine, excepting the one which he sent to Dr. Robison, so that we have no alternative hut to lay this before our readers.
Mr. Ilornblower had erected his engine for dif ferent manufactories, and for winding up coal at col lieries, and in 1796 he erected one at Messrs. Meux's brewery in London. Messrs. Boulton and Watt raised an action against Messrs. Hornblower and Mabel'ly for an infringement of the patent, and a de cision was given in the Court of Common Pleas in fa vour of Mr. Watt.