Chap. XXI. STEAM CONSIDERED AS A MOVING POWER.
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fluid which generates it, it does not possess a greater quantity of elastic force than thesame quantity of air. The latent heat of steam may be taken at about 950° ; steam at thetemperature of 212° contains 900° of heat, which is not detected by the thermometerin its gaseous state, so that the real quantity of heat may be taken at 950° added to 212°,or 1162°. If a quantity of steam be mixed with 5}, times its weight of water, at 32° oftemperature, the water will rise nearly to the boiling point : when water is boiled, itstemperature never rises above 212°, nor does the steam which is produced; the onlyeffect is in the more abundant supply of vapour, but if the water be confined in thevessel in which it is boiled, it as well as the steam may be brought to almost any tem-perature.
Steam Engine . — The high-pressure and the low-pressure arc the two kinds in ordinaryuse; the first is simple and composed of few parts, and well adapted for locomotiveengines, and other purposes where economy and portability are a consideration : the low-pressure engine is, however, not only more durable, but consumes less fuel, and is the mostefficient for all ordinary purposes. The high-pressure engine is sometimes called thenon-condensing, to distinguish it from the low-pressure engine, which is called the con-densing ; sometimes in one engine the properties of both are combined. The high-pressureengine now in use, the principles of which are more elementary and simple, was notbrought into practice until long after Mr. Watt had perfected the other.
High-pressure steam is produced by confining it within the boiler and increasing itstemperature and strength to a greater degree. The force of such steam is thus estimated :a pound weight is placed upon a hole on the top of the boiler, exactly an inch square;when the steam is powerful enough to lift the weight off’ the aperture on which it rests,its power is said to be one pound on the square inch : by placing a greater weight, agreater elastic pressure is produced, and 1000 pounds on an inch have been measured bythis means. The ordinary pressure given to engines of this kind varies from 15 to 120pounds per square inch: when 15 pounds is the pressure, it is called one atmosphere,or high-pressure steam of the elastic force of one atmosphere: 6 pounds of coals, em-ployed in converting 6 gallons of water into steam, is equal in power to the labour of aman for an entire day.
One of the simplest forms of application of high-pressure steam for the raising ofweights is by conducting the steam through a cylindrical tube, into which is inserteda movable piston, on the top of which is placed a weight: when the steam issues from theboiler, and passing under the piston, is strong enough to overcome the weight, the pistonwill rise to any required height; if the weight be equal to one atmosphere, and the area ofthe piston equal to a square inch, any increase in the pressure of the steam beyond oneatmosphere of elastic force will raise the weight.
The Atmospheric Engine is not worked by the direct arid immediate agency of steam,but by the atmosphere : the steam is here used only to make way for the atmosphere,and give eflect to its pressure. Engines of this description are usually constructed tocondense the steam in their own cylinders, and arc used for either raising water or coals:in construction they are simple, but when their cylinders are less than 24 inches indiameter, the consumption of fuel is considerably above the effect produced. In formingthe cylinder it is usual to make its diameter half its length, and the velocity should beabout 98 times the square root of the length of the stroke in feet per minute. The area ofthe steam passages is, as 4800, is to the velocity in feet per minute, so is the area of thecylinder, to the area of the steam passage. The number of cube feet of water required perminute for steam is found by multiplying the number of feet contained in the area of thecylinder, by half the velocity in feet, and that product by 1‘23 added to 1*4, divided bythe diameter in feet, and that quotient again divided by 1480. Twelve times the quantityof water required for the purposes of steam is necessary for its condensation, and the aper-ture by which it is injected should admit that quantity at every stroke that is made. Thearea of the jet aperture should be the 850th part of the area of the cylinder, and the con-ducting pipe four times the diameter of the jet.
To ascertain the power of an atmospheric engine, multiply 5*9 times the square ofthe diameter of the cylinder in inches, by half the velocity of the piston in feet per minute;the product so obtained expresses the effective power, or the number of pounds elevateda foot high per minute ; the horse-power is found by dividing by 33000. In a cylinderwith a diameter 72 inches, length of stroke 9 feet, and making 9 strokes per minute, it willbe shown by 9x9 = 81; then 5-9 x 72 x 72 x 81 = 2477433*6 lbs. raised a foot per minute,or 75 horse-power.
In the ordinary atmospheric engine, when the condensation takes place in the cylin-der, it is closed at the bottom and open at top, as represented at C : at S is a passagefor the steam at the bottom of the cylinder, with a valve or cock at V; at 1 is thepassage for the cold water, which condenses the steam, with a cock at D : at E is thepassage for the water which has been injected to run out; this passage has a valve