The earliest marine steam engines were very much on these lines, then, and were really a slightly modified form of land engine. But, as we shall soon come to refer to the more complicated type of engine, and to make use of other terms, it may not be out of place here to deal at once with the expression “horse-power,” which is used for the purpose of indicating the force which an engine is capable of developing. The origin of this expression is not without interest, and Sir Frederick Bramwell, Bart., F.R.S., D.C.L., in his entertaining article on the life of Watt in the “Dictionary of National Biography,” points out that Savery, to whom we have referred, was accustomed to calculate that where any machinery had to be driven by means of a single horse, it would entail a stock of three of these animals being kept, so that one should be able always to be at work. Thus supposing that the power exerted by six horses was necessary to drive a pump, and Savery made an engine capable of doing the same work by mechanical means, he would call it not a six horse-power engine, but an eighteen horse-power. Watt, however, did not credit his engine with the idle horses. He satisfied himself that an average horse could continue working for several hours when exerting himself so as to raise one hundredweight to a height of 196 feet in one minute, which is about equal to lifting 22,000 pounds one foot high in the same time, as the reader will find by simple arithmetic. But in order that no purchaser of his engines should have any ground for complaint, Watt went one step better, and determined that each horse-power of his engine should be capable of raising to a height of one foot, in one minute, not 22,000 pounds, but 33,000 pounds, or half as much again. And so to-day when we speak of an engine possessing such and such horse-power we still mean that it is equivalent to such a power as would raise 33,000 foot-pounds per minute. I make no apology for dwelling to such an extent on this point, but since at least one writer on steamships has seen fit to refer to this assessment of horse-power as being entirely arbitrary, and to admit in the same paragraph that he was altogether ignorant as to what power a horse was actually capable of producing, I have thought it not inappropriate to make the point clear in the mind of the reader. [Illustration: THE MARQUIS DE JOUFFROY’S STEAMBOAT. _From Mr. R. Prosser’s Pen-and-Ink Sketch in the Victoria and Albert Museum, South Kensington._]