(198.)

It is not usual to express the power of locomotive engines in the same manner as that of other engines by the term horse-power. Indeed, until the actual amount of resistance opposed to these machines, under the various circumstances in which they are worked, shall be ascertained with some degree of 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 major 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 [Pg406] has been here described is capable of evaporating 77 cubic feet of water per hour, while the early locomotives 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 railways. 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 Birmingham, was at the rate of 93·2 cubic feet per hour, and in returning the same distance 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 described.

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 evaporating power, are used on the Great Western Railway. In the autumn of 1838 experiments were made upon these engines by Mr. Nicholas Wood and myself, when we found that the most powerful engine on that line, the North Star, drawing a load of 1101⁄2 tons gross, engine and tender inclusive, at 301⁄2 miles an hour, evaporated 200 cubic feet of water per hour. The same engine drawing a load of 1941⁄2 tons at 181⁄2 miles an hour evaporated 141 cubic feet per hour, and when drawing 45 tons at 381⁄2 miles an hour evaporated 198 cubic 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 [Pg407] power evolved in the evaporation is allowed 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.