"tractive power" and "adhesive power" mean respectively the revolving power and the progressive power of the engine.

The Steam Fire-Engine.*

Steam fire-engines are simply hydraulic machines similar to steam-pumps, and the conditions involved in their employment are precisely the same. They are also steam-engines, with their machinery adapted to a special purpose, it being perfectly immaterial whether they are movable or stationary. Their means of locomotion is only a matter of convenience. The result of the working of the steam fire-engine may be measured by the hydraulic effect, and the power utilized may be determined by the quantity: of water delivered.

To determine the efficiency of steam fire-engines, it is necessary to note--first, the extreme vertical height and horizontal distance to which the water can be thrown; second, the volume or quantity delivered in a certain time; third, the total power consumed in performing that work.

Rule for finding the horse-power of steam-engines.

Multiply the area of the piston in inches by the average steam pressure in pounds per square inch; multiply this product by the travel of the piston in feet per minute,† and divide this product by 33,000; the quotient is the horse-power.

Rule for finding the horse-power of steam fire-engines.

Multiply the area of the piston in inches by the steam pressure in pounds per square inch; multiply this product by the travel of the piston in feet per minute, and divide this last product by 33,000; 7 of the quotient will be the horse-power.

Rule for finding the horse-power of a locomotive.

Multiply the area of the piston in inches by the pressure in

* For a full description of all the steam fire-engines in use at the present day, their peculiarities of design, construction, efficiency, etc., see Roper's "Hand-Book of Modern Steam Fire-Engines."

Which should never be less than 250 feet per minute; in fact, that should be the minimum piston speed for all classes of engines.

pounds per square inch; multiply this product by the number of revolutions per minute; multiply this by twice the length of the stroke in feet or inches; multiply this last product by 2 and divide by 33,000; the result will be the horse-power.

Rule for finding the horse-power of simple condensing engines. Multiply the area of the piston in inches by the mean effective pressure in pounds per square inch; multiply this product by the velocity of the piston in feet per minute; multiply the atmospheric pressure in pounds per square inch on the bucket of the air-pump by its velocity in feet per minute; subtract the last product from the second, and divide the remainder by 33,000; the quotient will be the horse-power of the engine.

In estimating the horse-power of steam-engines by the foregoing rules, not more than two-thirds of the boiler pressure should be taken; as the analysis of a large number of indicator diagrams shows that the average pressure in the cylinders of slide-valve engines rarely, if ever, exceeds two-thirds of the boiler pressure. This difference is due to the reduction caused by the pipes, stopvalves, and the condensation in the pipes, cylinder, etc.

Rule for finding the horse-power of a steam-engine by indicator diagrams.

Multiply the area of the piston by its travel in feet per minute, and divide by 33,000; the quotient will be the value of one pound of mean effective pressure, which, if multiplied by the total mean effective pressure, as shown by the card, will give the indicated horse-power.

Example.— Area of piston, 113.

Travel of piston in feet per minute, 3331.

113 × 3331

11

33,000

= 1.141 horse-power value of 1 lb. M. E. P. 36 M. E. P. as shown by the card.

6846

3423

41.076 horse-power.

The above cut shows a section of the cylinder, piston, and steam-chest of an ordinary slide-valve engine; a represents the cylinder; b, the piston; c, the piston-rod; o o, recesses in the cylinderhead; kk, steam-ports; l, exhaust cavity in the valve-seat; n, exhaust opening in valve-face; e, valve; f, valve-rod; dd, steam-chest; m, bonnet of steam-chest, and h h, clearance.

The term clearance is understood by engineers to mean the unoccupied space between the piston- and cylinder-heads when the crank is at the dead-centre; but it also applies to the space between the cylinder and the face of the valve or valves, either slide or poppet. The amount of clearance of any engine affects its economy; and if the clearance is small, the engine will be more economical than if large; a certain amount is an absolute necessity. It is, therefore, an object of importance, in point of economy, to have the valve-face as near the base of the cylinder as possible. In this lies one of the most important features of the Buckeye,

Brown, Putnam, Woodruff and Beach, etc., and, in fact, all engines of the Corliss type. The clearance varies with different builders, and in different engines from 1 to 10 per cent. of the cubic contents of the cylinder.

The clearance is often as high as fifteen per cent., in some oldfashioned long stroke, slide-valve engines. This arose from a mis-conception, at the time they were designed, of the waste the large clearance would occasion, and is, perhaps, in many instances, due to the caprice of the inventor of some patent piston, who made his piston-rings of less depth than the original designs, thus increasing the space between the piston- and cylinder-heads, when the crank is at the dead-centre. There are even cases to be met with, where the old fashioned, hemp-packed piston has been replaced by metallic packing of not more than half its depth, without any means being taken to fill up the spaces at each end of the cylinder. Now, providing that the clearance is fifteen per cent. of the cubic contents of the cylinder, and that the engine makes from one hundred and fifty to two hundred strokes per minute for ten hours, it may easily be seen how enormous the waste must be. The quantity of fuel that might be saved by replacing such an engine by one in which the clearance would be reduced to a minimum, would more than pay for the latter in five years. Persons employing steam-power, or intending to purchase steam-engines, should pay attention to the foregoing fact.

As the clearance space is generally irregular in form, particularly in slide-valve engines, it is somewhat difficult to calculate the exact cubic space. The most accurate method of ascertaining the exact amount of the clearance is to place the crank at the dead-centre, and fill the space with water up to the face of the valve (the quantity of water being previously weighed or measured). Then deduct the amount remaining in the vessel from the whole, and the remainder will be the quantity contained in the clearance in cubic inches or gallons, as the case may be.

The Woodruff & Beach Automatic Cut-Off High-Pressure Engine.

The cut on the opposite page represents the Woodruff & Beach high-pressure automatic cut-off engine. Fig. 1 shows a section of the cylinder-valves, steam passages, and exhaust passages. Fig. 2 is a back view of the cylinder, steam-chests, valve-gear, etc. With the exception of the Corliss, it is the oldest variable cut-off engine in the country, and one that has undergone fewer changes in its mechanism than any other. Those who remember it thirty years ago, will fail, at the present day, to discover much difference from its general appearance. For more than a quarter of a century it has successfully competed with such engines as the Corliss, and it has always sustained a high rating in the scale of comparative merit. The bed-plate, as will be observed, is of the ordinary box O. G. pattern, to which the cylinder-guides and pillow-blocks are bolted and dowelled in such a manner that the possibility of their getting out of line is entirely obviated.

The steam-valves, which are of the double poppet form with bevelled faces and seats, are located at the back of the cylinder at each end, horizontal with its axis. Their stems project inward, and, owing to the peculiar shape of the cam which gives the motion, the opening and closing is done very quickly and almost noiselessly. They have independent adjustments, so that the steam lead may be varied to meet any requirement without interfering with the rest of the valve-gear. The power required to work the valves in these engines is very slight, and as the camlug and the ends of the valve-stems are made of hardened steel, they show no perceptible sign of wear after years of use.

The exhaust-valve, which is cylindrical in form and has a very convenient arrangement for taking up the wear and preventing leakage, is placed at the bottom of the cylinder, and communicates with it by its own ports or passages, which are entirely sepa rate from those of the steam-valve. An equilibrium of pressure is maintained by the exhaust taking place through the interior