Page:The Development of Navies During the Last Half-Century.djvu/269

 been the first ship successfully fitted with compound engines for the Royal Navy; for though the 'Constance' preceded her, that ship’s machinery was a constant source of extreme worry and anxiety to those responsible for the charge of it, while the 'Pallas' in this respect never gave any trouble at all. The 'Pallas' had only two cylinders, instead of six, of unequal volume, one being four times the size of the other. The steam was admitted at high pressure, 60 lbs., into the small cylinder, and thence passed into the larger one, which it of course filled by its expansion. This is the whole principle of compound engines. She had surface condensers, and there is no doubt, for the horse power produced, she was very economical in fuel. The boilers were fitted with superheaters, a series of tubes at the base of the funnel through which the steam passed with the object of drying it and surcharging it with heat — a contrivance that was always looked upon with distrust by naval engineers and has long ago passed into oblivion. Her speed was 13.4 knots, and she was a handy and essentially comfortable little ship.

The compound principle, as introduced by Messrs Humphrys & Tennant for the 'Pallas,' was, but at an interval, adopted for most of the new ships in the navy, until superseded by triple-expansion, to a consideration of which we shall come by-and-by. With compound engines came as a natural consequence surface condensers, and the general use of steam of not less than 60 lbs. pressure. This latter innovation brought about an entire revolution in the shape of ships’ boilers.