106 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1907. 
good vacuum, attaining a velocity of about 4,500 feet per second, and 
allowed to impinge on a stationary brass blade, the blade will be cut 
through in a few hours, and the hardest steel will be slowly eroded. 
The action seems to be the result of the intense local pressure from 
the bombardment of the drops, which may exceed 100 tons. 
Owing to the receding velocity of the blades from the blast, and 
consequently reduced striking velocity, the erosion of the blades in 
impact turbines is much reduced, and in compound turbines there is 
complete immunity from such erosion. 
It may be asked, how is it that the steam turbine in the larger 
sizes is more economical in steam per horsepower development than 
the best triple or quadruple expansion reciprocating engine? The 
reason is that all large steam turbines are able to take full advantage 
of the whole expansive energy of the steam, even when expanding to 
the very attenuated vapor densities produced by the best condensers. 
Tt is indeed easy to construct the low-pressure portion of the turbine 
to deal effectively with the very attenuated vapor, whereas the re- 
ciprocating engine, from its nature, can only take full advantage of 
about two-thirds of the whole range of expansion, and is unable to 
deal usefully with very low vapor densities—the low-pressure cyl- 
inders can not (because of structural difficulties) be made large 
enough, and the last part of the expansion has to be allowed to run 
to waste. 
The growth in size of the turbine is perhaps interesting. The first 
practical steam turbine, constructed in 1884, was of 10 horsepower. 
By 1892 the largest size for driving dynamos had reached 200 horse- 
power. It has been continuously increasing, and has now reached 
12,000 horsepower in one unit driving one alternating dynamo. 
In 1894 the Turbinia, of 2,000 I. H. P., was commenced. The 
diagram (fig. 5) shows her low-pressure and reversing turbine. The 
L. P. turbine is 3 feet in diameter. 
The King Edward was built in 1902, 9,300 I. H. P., and the 
diagram shows one of her L. P. turbines and reversing turbine in one 
casing, to the same scale. 
In 1903 The Queen, of 9,000 I. H. P., commenced to ply between 
Dover and Calais. The diagram shows one of her L. P. and reversing 
turbines. 
In 1905 the Allan liners Virginian and Victorian, of 12,000 I. H. P.. 
went on service between Liverpool and Canada. The diagram shows 
one of the L. P. and reversing turbines, which is 10 feet in diameter 
and 35 feet in length; and in last December the Carmania, of 30,000 
tons displacement and 20,000 horsepower, commenced to ply between 
Liverpool and New York. The diagram shows her L. P. turbine, 
which is 14 feet in diameter. 
