About the time when the Suez Canal was opened the practical adoption of the compound engine was taking place in the mercantile marine. This idea had been introduced about 1856 by Messrs. Randolph Elder and Company, and had been installed in the ships of the Pacific Steam Navigation Company. In explanation of this system we may say at once that its great advantage lay in the fact that it reduced the coal consumption to just about half of what it had been hitherto in the most economical engines. The principle is based on the fact that steam possesses elastic properties which can be put to excellent use. Put simply, the compound engine allows the steam to enter one cylinder at high pressure, and, after it has moved the piston, escapes into one (or more) cylinders of larger size, where it does its work by direct expansion, and so much more work is done at little expense. The expression “triple expansion,” which frequently confronts the reader interesting himself in steamships, simply means that the steam is expanded one stage further. Quadruple expansion is the same idea pushed still another stage. When about twenty years ago the triple expansion system was brought in, the steam pressures were increased from 125 lb. to 160 lb. per square inch, and so the coal consumption was reduced also. But the triple expansion had been preceded by the compound and the low pressure engine, just as it was followed by the quadruple. The opening of the Suez Canal was not devoid of side issues, for it took away that monopoly which the P. and O. had enjoyed, since the world’s steamships now poured in and began to go eastward and back again. There was difficulty with the Post Office, who refused to allow the Canal route for the conveyance of mails, on the ground that it was not so suitable as the Egyptian Railway, and it was not until 1888, when the charge for carrying the mails had been reduced by nearly £100,000 a year, that the accelerated mails sent via Brindisi were transferred to the Canal route, although the heavy mails had already been carried by it. But the P. and O. were unlucky in another way. The _Mooltan_, their first ship to be installed with the compound engine, in 1860, had proved such a success that several other steamers of the line were thus fitted, but the result was disappointing. Although it was quite clear that this type of engine made for economy, yet it was found unreliable, and in some cases had to be replaced by less complex machinery.