% MECHANICS AND USEFUL ARTS. 51 



the size of an engine, but simply by increasing the pressure of the 

 steam, the power of an engine may be greatly increased ; that is, 

 the load remaining constant, the speed of the piston may be in- 

 creased, the number of strokes may be increased, and consequent- 

 ly the work done per minute will be increased also. Hence it is 

 difficult to apply a limit to the power obtainable from the smallest 

 cylinder, provided the boiler be large enough to evaporate the 

 increased quantity of water, and strong enough to resist the 

 increased bursting pressure. In fact, no size of cylinder can be 

 reckoned as having a particular power, since the power depends 

 not on size but on strength. Nevertheless, in modern engineering, 

 the term horse-power refers rather to the size of the cylinder than 

 to the power exerted ; and the value of this unit has undergone 

 many changes, so that in a modern engine a horse-power may 

 imply 52, or 60, or 66,000 i^ounds, one foot high, per minute. 



The plan now adopted for ascertaining the performances of dif- 

 ferent engines, is by an instrument called an indicator. This con- 

 sists of a small cylinder, fitted with a piston, which is pressed 

 down by a spring. By the height to which this piston rises 

 against the spring the steam pressure within the cylinder is indi- 

 cated ; and the number of pounds pressure on the square inch, 

 niultij^lied into the number of square inches in the area of the 

 cylinder, and by the number of feet travelled through by the 

 piston per minute, gives the impelling power; deduct, in large 

 engines, about one-tenth for friction, and the remainder is the effi- 

 cient moving power, which, divided by 33,000, gives the actual 

 horse-power. 



ADVANTAGES OF SUPEEHEATED STEAM. 



Mr. H. W. Bulkley of New York makes the following com- 

 munication in the " Journal of the Franklin Institute" for Octo- 

 ber, 1866. " Supei-heated " steam, or steam which has received 

 an increase of temperature without increase of weight, by the 

 direct application of heat, has enemies who stoutly maintain that 

 no benefit can be derived from the superheating, as the steam has 

 its maximum efiiciency as soon as generated. 



The fallacy of such statements is evident on reflection, and 

 plainly shows that those advancing and upholding them have 

 neither practical acquaintance with the subject, nor have given it 

 serious thought. It is clear that, as the greater part of the steam 

 generated in boilers is obliged to jiass through the water above it, 

 on its way to the steam-pipe, it must unavoidably carry with it 

 much water in the form of spray, mechanically combined, and held 

 in suspension. When boilers " foam,'^ this operation is greatly 

 increased by unnatural causes, the delivery of spray becoming so 

 great as to seriously inconvenience the engine, and endanger its 

 safety, as well as that of the boiler. And, in boilers properly con- 

 structed and carefully operated, which may be supposed to work 

 dry steam, much more water than is generally conceived is con- 

 stantly carried over with the steam ; and this delect cannot be en- 

 tirely remedied, even by the most judicious arrangement of " dry 

 pipes," steam-drums, etc. What,' then, becomes of this water 



