square inches. The square root of this is twenty-three inches, which will be the side of a square chimney. Or multiply 533 by 1.27 and extract the square root for the diameter of a circular one.

But in either case I would advise to build a chimney of double the area, or 1066'4 area, that is, make the side of the square thirty-three inches.

In this rule it is supposed that the engine is done in the best manner, and worked with the best coals; that is, one requiring only from nine pounds to eleven pounds of coal per hour, for each horse power, of an engine above ten horses' power. But where fourteen or sixteen pounds of coal per hour is necessary, the flue should be increased in direct proportion to the quantities to be consumed. See the mode of finding the rule in (art. 168.)

276.-When wood is used for fuel, it affords a much larger quantity of smoke, but it is also much lighter, and about one and a half times the area necessary for coals will be sufficient.

277.-The same rules may be applied to high pressure engines; taking the cubic feet of water per hour, or the one-eleventh part of the pounds of coal per hour, instead of the number of horses' power.

278.-The engine chimneys for steam boats and steam carriages are circular, and should not be larger than is absolutely required to give effect to the fuel. This will be about obtained when the square of the diameter is equal to 90 multiplied by the horses' power, and divided by the square root of the height in feet.

But here it must be remarked, that where a chimney is less than about forty or fifty feet in height, the smoke must be allowed to rise at a much higher temperature. It must not therefore be allowed to cool too much by giving its heat to the boiler, otherwise there will be a want of draught. Hence, in low chimneys, the fuel will not produce its full effect. Different modes of finishing chimney tops, are shewn in Plate I. The least expensive is one of the form of an Egyptian obelisk, and it offers least obstruction to the wind.

Of the Condensation of Steam.

279.-When any substance or body colder than steam itself is put in contact with it, the steam condenses till the temperature of the cold body becomes the same as that of the steam; or till the whole mass of steam be condensed to a degree of elasticity corresponding to the temperature to which the cold body is raised by the heat of the steam. The greater the quantity of the cold body the less its temperature will be raised, and also the colder it is the more the elastic force will be reduced. Hence, to reduce the elastic

force of steam as low as possible, the coldness and the quantity of the cooling body should be as great as possible.

280.-Any cold body condenses steam, but that it may be effectively done the body should be capable of presenting a large quantity of surface, and be a good conductor of heat; as when power is to be obtained by condensation the more rapid the condensation is the more power is obtained. It may be easily proved that if steam were so condensed as to lose only equal degrees of elastic force in equal times during the action, half the power would be lost. (See art. 294.) This is the cause of the failure of every method of slow condensation; it cannot be too prompt, unless a sacrifice of power is made in some other way to gain that promptness, and to which the effect gained by condensation is not equivalent.

281.-Water has been found the most effective cold body for condensation; it has great specific heat, perhaps greater than any other body; it is a rapid conductor of heat, and in a jet it applies an immense proportion of cooling surface to the steam.

Now since water is frequently difficult to be procured of a low temperature, and sometimes not in sufficient quantity, it becomes important to inquire what effect is produced by given proportions at given temperatures.

282.-The weight of the water, W, required for condensation, multiplied by the quantity at its temperature is raised, gives the heat it absorbs; and, in the steam engine, where the operation is repeated in the same vessels, and at the same temperatures, the excess of the temperature of the steam T-x above that to which the condensing water is raised added to 1000, and the sum multiplied by the weight w of the steam, must be equal to the heat absorbed by the condensing water. That is

283.-When the temperature of the condensed water is equal to the temperature of the steam, the quantity of water would be equal to that which simply reduces the steam to water without change of temperature; or

But in this case no effect would be obtained. Any greater quantity of cold water reduces

To make this equation general, let s be the specific heat of the condensing body, and C the heat of conversion, and s the specific heat of the body in vapour, then W s ( at ) = ws' (C + T − x ).

the elastic force, but it must be so far reduced as to render the accession of power more than equivalent to that required to work an air pump, and cover the expense of a supply of water, and the extra cost of the engine.

284.-In low pressure steam T = 220°, and t may be taken at 52° the mean temperature, and if the temperature of the condenser be 100°, then

That is 23 times the quantity of. the water required for steam, will be the quantity of water necessary for condensation. And since a cubic inch of water produces about a cubic foot of steam of the rarity, it is the cylinder of an engine working at this temperature, and one-tenth being added for each foot of the capacity of the stroke, 23 x 1.1 is 25} inches for each foot of the contents of the stroke of the cylinder.*

If a 130°, it requires of cold water only fourteen times the weight of the steam to condense it, and for 120° it requires 16-2 times the weight.+

The force of steam at 100° is 2.08 inches of mercury, its force at 130° is 4.81 inches; consequently, the gain of power is 2·73 inches, or about one in thirteen, by condensing at the lower temperature.

If the temperature of the cold water be 70°, and of the condenser 130°, then we find cold water eighteen times the weight of the steam will condense it; and that it requires thirtyseven times the weight to condense at 100°, when the cold water is at 70°.

285.-From these equations the comparative effects of different temperatures may be calculated, and the economy of using or sparing water will be known and acted upon, instead of the usual method of endeavouring to get the greatest power of the steam in places where water is expensive.

When steam is of considerable density it does not condeuse freely; the reason is obvious, the same surface of injection water acting on steam of greater density, and consequently containing a greater proportion of heat, it abstracts the heat more slowly. To avoid this the condenser should be so large that the steam may expand to the bulk corresponding to a pressnre not greater than about one atmosphere and a half. But it is better to make the steam act expansively in the cylinder, by Watt's method, (art. 27.) or expand in a second cylinder by Hornblower's method, (art. 32.)

When a lower temperature than 180° cannot be obtained by condensation, it is not worth the extra expense, and at 180° we have for low pressure steam

Mr. Watt says a wine pint, or 283 inches is "amply sufficient." Robison's Mech. Phil. Vol. II.

+ The usual temperature is about 120o, or just what the hand can bear.

nearly; or eight times the quantity of water required for steam will be necessary to condense it.

286.-These computations apply to where condensation is made in a separate vessel, the first idea of which we owe to Mr. Watt. When the condensation is made within the cylinder, the metal of the cylinder has to be cooled down to the temperature of condensation as well as the steam, and a large proportion of the steam is lost in heating it again at each stroke. The means of obtaining a maximum of useful effect from condensing in that manner has been shewn, (art. 165.)

287.-To find the quantity of water for injection into an engine condensing in the cylinder, the formula is the same as when a separate condenser is used, the difference being in the quantity of steam required; and the water for condensation is greater than when Watt's condenser is employed by

14 i (Tx)
x- t

for each stroke, when i is the weight of the mass of iron contained in the cylinder. 288.-The following tabular view of the modes of condensation may perhaps present it in a clearer view to the reader than any other kind of concluding summary.

SECTION IV.

OF THE MECHANICAL POWER OF STEAM, AND THE NATURE, GENERAL PROPORTIONS, AND CLASSIFICATION OF STEAM ENGINES.

289. The force of steam when confined, according to its density and temperature, and the circumstances which affect its motion, having been considered, our next object is to investigate the power of steam to produce useful effect, and in this purpose I am desirous of proceeding with the simplicity and fulness this important subject requires.

Of the Power of Steam, and the Modes of obtaining it.

290.-The generation or production of steam, it has been shewn, takes place on the application of heat. Conceive a cylindric vessel, A B, to be placed in a vertical position, with a given depth of water in it; and an air-tight piston on the water balanced by a weight equal to its own weight and friction In this state let heat be applied to the base, A C, then as the water becomes converted into steam, of slightly greater force than the atmospheric pressure, the piston will rise till the whole of the water be in the state of steam. It will be remarked, that the generation of this steam of atmospheric elastic force affords no power, the motion being barely produced; it has simply balanced the column of atmospheric air, and excluded it from a given height of the cylinder.

291.-By Condensation.-But in this state of things if the steam be suddenly condensed into water again, it is obvious that the piston will be impelled by a force equal to the pressure of the atmosphere