562 THE STEAM-ENGINE. 



precision, it is impossible that their power or efficiency can be estimated with ' 

 any tolerable degree of approximation. The quantity of water evaporated, 

 and passed in steam through the cylinders, supplies a mijor limit to the power 

 exerted ; but even this necessary element for the calculation of the efficacy 

 of these machines has not been ascertained by a sufficiently extensive course 

 of observation and experiment. Mr. Stephenson states that the engine which 

 has been here described is capable of evaporating 77 cubic feet of water per 

 hour, while the early locomotive could only evaporate 16 cubic feet per hour. 

 This evaporation, however, is inferior to that which I have ascertained myself 

 to be produced by engines in regular operation on some of the northern rail- 

 ways. In an experiment made in July, 1839, with the Hecla engine, I found 

 that the evaporation in a trip of ninety-five miles, from Liverpool to Birming- 

 ham, was at the rate of 93.2 cubic feet per hour, and in returning the same dis- 

 tance it was at the rate of 85.7 cubic feet per hour, giving a mean of 89 cubic 

 feet per hour nearly. The Hecla weighed 12 tons ; and its dimensions and 

 proportions corresponded very nearly with those of the engine above de- 

 scribed. 



In a course of experiments which I made upon the engines then in use on 

 the Grand Junction railway in the autumn of 1838, I found that the ordinary 

 evaporating power of these engines varied from eighty to eighty-five cubic feet 

 per hour. 



Engines of much greater dimensions, and consequently of greater evapora- 

 ting power, are used on the Great Western railway. In the autumn of 1838, 

 experiments were made upon these engines by Mr. Nicholas Wood and my- 

 self, when we found that the most powerful engine on that line, the North 

 Star, drawing a load of 110| tons gross, engine and tender inclusive, at 30^ 

 miles an hour, evaporated 200 cubic feet of water per hour. The same engine 

 drawing a load of 194^ tons at 181- miles an hour, evaporated 141 cubic feet 

 per hour, and when drawing 45 tons at 38 J miles an hour, evaporated 198 cu- 

 bic feet of water per hour. 



It has been already shown that a cubic foot of water evaporated per hour 

 produces a gross amount of mechanical force very little less than two horse 

 power, and consequently the gross amount of mechanical power evolved in 

 these cases by the evaporation of the locomotive boilers will be very nearly 

 twice as many horse-power as there are cubic feet of water evaporated per 

 hour. Thus the evaporation of the Hecla, in the experiments made in July, 

 1839, gave a gross power of about one hundred and eighty horses, while the 

 evaporation of the North Star gave a power of about four hundred horses. In 

 stationary engines about half the gross power evolved in the evaporation is al- 

 lowed for waste, friction, and other sources of resistance not connected with 

 the load. What quantity should be allowed for this in locomotive engines is 

 not yet ascertained, and therefore it is impossible to state what proportion 

 of the whole evaporation is to be taken as representing the useful horse- 

 power. 



The great uniformity of resistance produced by the traction of carriages 

 upon a railway is such as to render the application of steam-power to that | 

 purpose extremely ad vantage ous. So far as this resistance depends on me- < 

 chanical defects, it is probably rendered as uniform as is practicable, and in | 

 proportion to the quantity of load carried is reduced to as small an amount as 

 it is likely to attain under any practicable circumstances. Until a recent pe- 

 riod this resistance was ascribed altogether, or nearly so, to mechanical causes. 

 The inequality of the road-surface, the friction of the axles of the wheels in 

 their bearings, and the various sources of resistance due to the machinery of 

 the engine, being the principal of these resistances, were for the most part in- 



