[Illustration: Model of the Persia and Scotia.]
1830 +------------------------------------ 9 lbs. | 1840 +---------------------- 5-1/2 „ | 1850 +---------------- 4 „ | 1860 +------------ 3.1 „ | 1870 +---------- 2.6 „ | 1880 +-------- 2.2 „ | 1886 +------ 1.5 „ and | since|
Diagram showing Decrease in Expediture of Coal per indicated Horse-power per hour based on Good Average Practice
1830— 2 @ 3 lbs. +-- 1835— 5 lbs. +-- 1840— 8 „ +-- 1845—10 „ +-- 1850—14 „ +--- 1855—21 „ +---- 1860—30 „ +----- 1865—40 „ +------- 1870—50 „ +--------- 1875—60 „ +----------- 1880—70 „ +------------- 1882—80 „ +--------------- 1885-6—150 @ 180 lbs. +-------------------------------- and since
Diagram showing increase in Steam-pressures based on good average Practice
How great an advantage she was upon their first ship will be seen by the following comparison:
Britannia. Persia. Coal necessary to steam to New York 570 tons 1,400 tons Cargo carried 224 „ 750 „ Passengers 90 250 Indicated power 710 3,600 Pressure per square inch 9 lbs. 33 lbs. Coal per indicated horse-power per hour 5.1 „ 3.8 „ Speed 8.5 knots 13.1 knots
Thus, for two and a half times the quantity of coal nearly three and a half times the cargo was carried, and nearly three times the number of passengers. This result was due partially to increased engine efficiency, and partially to increased size of ship; and thus to a relative reduction of the power necessary to drive a given amount of displacement.
The Scotia was almost a sister ship to the Persia, slightly exceeding her in size, but with no radical differences which would mark her as an advance upon the latter. She was the last of the old régime in the Atlantic trade, and the same year in which she was built saw the complete acceptance by the Cunard company of the newer order of things, in the building of the iron screw steamer China, of 4,000 tons displacement, with oscillating geared screw engines of 2,200 indicated horse-power, with an average speed of 12.9 knots on a daily expenditure of 82 tons of coal. She was the first of their ships to be fitted with a surface condenser. The Scotia had been built as a paddle steamer rather in deference to the prejudices of passengers than in conformity to the judgment of the company, which had put afloat iron screw ships for their Mediterranean trade as early as 1852 and 1853.