greatly exceeded in the future. As there is no known and definite limit to the economical increase of speed, and as the limit set by practical conditions is continually being set farther back as the builder acquires greater skill and attains greater accuracy of workmanship and the power to insure greater rigidity of parts and durability of wearing surfaces, we must anticipate a continued and indefinite progress in this direction-a progress which must evidently be of advantage, whatever may be the direction that other changes may take.

It is evident that this adaptation of the steam-engine to great speed of piston is the work now to be done by the engineer. The requisites to success are obvious, and may be concisely stated as follows:

1. Extreme accuracy in proportions.

2. Perfect accuracy in fitting parts to each other.

3. Absolute symmetry of journals.

4. Ample area and maximum durability of rubbing surfaces.

5. Perfect certainty of an ample and continuous lubrication.

6. A nicely calculated and adjusted balance of reciprocating parts.

7. Security against injury by shock, whether due to the presence of water in the cylinder or to looseness of running parts.

8. A "positive-motion " cut-off gear.

9. A powerful but sensitive and accurately-working governor determining the degree of expansion.'

1 The author is not absolutely confident on the latter point. It may be found more economical and satisfactory, ultimately, to determine the point of cut-off by an automatic apparatus adjusting the expansion gear by reference to the steam-pressure, regulating the speed by attaching the governor elsewhere. The author has devised several forms of apparatus of the kind referred to. 32

10. Well-balanced valves and an easy-working valve-gear. 11. Small volume of "dead-space," or or "clearance," and properly adjusted "compression."

It would seem sufficiently evident that the engine with detachable ("drop") cut-off valve-gear must, sooner or later, become an obsolete type, although the substitution of springs or of steam-pressure for gravity in the closing of the detached valve may defer greatly this apparently inevitable change. The "engine of the future" will not probably be a "drop cut-off engine."

As regards the construction of the engine as a piece of mechanism, the principles and practice of good engineering are precisely the same, whether applied in the designing of the compound or of the ordinary type of steam-engine. The proportioning of the two machines to each other in such manner as to form an effective whole, by procuring approximately equal amounts of work from both, is the only essential peculiarity of compound-engine design which calls for especial care, and the method of securing success in practice may be stated to be, for both forms of engines, as follows:

1. A good design, by which is meant

a. Correct proportions, both in general dimensions and in arrangement of parts, and proper forms and sizes of details to withstand safely the forces which may be expected to come upon them.

b. A general plan which embodies the recognized practice of good engineering.

c. Adaptation to the specific work which it is intended to perform, in size and in efficiency. It sometimes happens that good practice dictates the use of a comparatively uneconomical design.

2. Good construction, by which is meant—

a. The use of good material.

b. Accurate workmanship.

c. Skillful fitting and a proper "assemblage" of parts.

3. Proper connection with its work, that it may do that work under the conditions assumed in its design.

4. Skillful management by those in whose hands it is placed.

In general, it may be stated that, to secure maximum economical efficiency, steam should be worked at as high a pressure as possible, and the expansion should be fixed as nearly as possible at the point of maximum economy for that pressure. In general, the number of times which the volume of steam may be expanded in the standard singlecylinder, high-pressure engine with maximum economy, is not far from P, where P is the pressure in pounds per square inch; it rarely exceeds 0.75 VP. This may be exceeded in double-cylinder engines. It is even more disadvantageous to cut off too short than to "follow' too far." With considerable expansion, steam-jacketing and moderate superheating should be adopted, to prevent excessive losses by internal condensation and reëvaporation; and expansion should take place in cylinders in series, to avoid excessive weight of parts, irregularity of motion, and great loss by friction, as well as to insure economy.

To secure this vitally important economy, it is advisable to seek some practicable method of lining the cylinder with a non-conducting material. This plan, as has been seen, was adopted by Smeaton, in constructing Newcomen engines a century ago. Smeaton used wood on his pistons, and Watt tried wood as a material for steam-cylinder linings. That material is too perishable at temperatures now common, and no metal has yet been substituted, or even discovered, which answers the same purpose. The loss will also be reduced by increasing the speed of rotation and velocity of piston. Where no effectual means can be found of preventing contact of the steam with a good absorbent and conductor of heat, it will be found best to sacrifice some of the efficiency due to the change of state of the vapor, by superheating it and sending it into the cylinder

at a temperature considerably exceeding that of saturation. With low steam and slowly-moving pistons, it is better to pursue the latter course than to attempt to increase the efficiency of the engine by greater expansion.

External surfaces should be carefully covered by nonconductors and non-radiators, to prevent losses by conduction and radiation of heat. It is especially necessary to reduce back-pressure and to obtain the most perfect vacuum possible without overloading the air-pump, if it is desired to obtain the maximum efficiency by expansion, and it then becomes also very necessary to reduce losses by "deadspaces" and by badly-adjusted valves.

The piston-speed should be as great as can be sustained with safety.

Good engines should not require more than W =

where W = the weight of steam per hour and

180

200

VP'

per horse

power; the best practice gives about W = in large en

VP

gines with dry steam, high piston-speed, and good design, construction, and management.

The expansion-valve gear should be simple. The point of cut-off is perhaps best determined by the governor. The valve should close rapidly, but without shock, and should be balanced, or some other device should be adopted to make it easy to move and free from liability to cutting or rapid wear.

The governor should act promptly and powerfully, and should be free from liability to oscillate, and to thus introduce irregularities which are sometimes not less serious than ̧ those which the instrument is intended to prevent.

Friction should be reduced as much as possible, and careful provision should be made to economize lubricants as well as fuel.

The Principles of Steam-Boiler Construction are exceedingly simple; and although attempts are almost daily made

to obtain improved results by varying the design and arrangement of heating-surface, the best boilers of nearly all makers of acknowledged standing are practically equal in merit, although of very diverse forms.

In making boilers, the effort of the engineer should evidently be:

1. To secure complete combustion of the fuel without permitting dilution of the products of combustion by excess of air.

2. To secure as high temperature of furnace as possible. 3. To so arrange heating-surfaces that, without checking draught, the available heat shall be most completely taken up and utilized.

4. To make the form of boiler such that it shall be constructed without mechanical difficulty or excessive expense.

5. To give it such form that it shall be durable, under the action of the hot gases and of the corroding elements of the atmosphere.

6. To make every part accessible for cleaning and repairs.

7. To make every part as nearly as possible uniform in strength, and in liability to loss of strength by wear and tear, so that the boiler when old shall not be rendered useless by local defects.

8. To adopt a reasonably high "factor of safety" in proportioning parts.

9. To provide efficient safety-valves, steam-gauges, and other appurtenances.

10. To secure intelligent and very careful manage

ment.

In securing complete combustion, the first of these desiderata, an ample supply of air and its thorough intermixture with the combustible elements of the fuel are essential; for the second-high temperature of furnace-it is necessary that the air-supply shall not be in excess of that absolutely