Seeing, however, that the power required for a certain speed varies with the cube root of the displacement squared, the proportion of power to tonnage will decrease considerably with the increase in the size, so that if, instead of the steamer above referred to of 4,100 tons, one were taken of 8,200 tons, the I. H.-P. for 15 knots—all other things remaining the same—would be very little more than 5,000; _i. e._, with a ship of twice the size the increase of engine-power is only forty-seven per cent. The carrying capacity and consequent earning power of such a boat is immeasurably more than that of the small one. The larger ship will, moreover, make better passages, and generally be much more economical in working, as the officers and crew will not very largely exceed that of the smaller vessel.
It was, however, owing to the more economical engine that advances in speed were rendered possible, and this is seen by referring back to the original ship, and supposing that instead of engines burning 4 pounds of coal per I. H.-P., it had ones consuming only 2-1/2 pounds per I. H.-P. in which case the expenditure on the voyage would be reduced from 1,618 tons to 1,004 tons; so that 600 tons more cargo could be taken and the cost of 600 tons of fuel per voyage saved. This was actually the case on the substitution of compound for old-fashioned low-pressure jet-injection engines fitted to the Cunard Company’s steamers as late as 1862, when their largest, fastest, and most improved steamer, the Scotia, was put on the service. But it was not until many years after the advent of the Scotia that such economic engines were in general use on the Atlantic, and it was only in 1874-75, when the Inman Company and White Star Company placed steamships having these engines in competition with the old-fashioned ones, that the day of the latter was gone.
[Illustration: Passenger Steamer Princesse Henriette at Full Speed—24-1/2 miles per hour.
(Built by William Denny & Co., Dumbarton.)]
The first pioneers of steamship construction were apparently satisfied to find their efforts result in some motion, for we find exultation rather than disappointment in the accounts extant of Patrick Miller’s experiments with a small steamer on a Scotch canal in the year 1787; and later, in 1789, when, with a larger and better boat and machinery, he was able to obtain a speed of 7 miles an hour (equivalent to 6.07 knots[12]) it was deemed a great achievement; later still, in 1807, Fulton’s first attempt with the steamship Clermont, in a run from Albany to New York and back, the average speed was only 5 miles an hour. In those days so long as a steamer was able to face wind and tide she was deemed a success. The competition of steamers in early times (when there was any) was with sailing ships, or with land conveyances whose maximum rate would be 10 miles an hour, and that effected at considerable cost in horse-flesh. It is, however, true that sailing ships did then, and can now, sail, under favorable circumstances, at very much higher rates than we have just mentioned, and even as much as 15 knots can be obtained with one of fine lines with a favoring wind; but a sailing ship is not always free to traverse the shortest distance from port to port, and even when wind and weather permit of this, the _average_ speed falls far below 15 knots with the best-designed vessels; hence if a steamer could do 9 knots she would make shorter passages than any sailer; and from the nearer approach to uniformity in the time occupied, passengers were attracted to steamships, and the passenger sailing vessel, except for very long voyages, became a thing of the past.