CXXXII. S. H. BARRACLOUGH. 
sible to classify them, and so recourse must be had to a system of 
trial and error in which an examination is made of two or three 
types of boiler and engine, which experience indicates to be the most 
suitable. The adoption of such a system is greatly assisted by 
carefully prepared tables giving the cost of steam power per H.P. 
per annum under a great variety of typical sets of working con- 
ditions. One of the most elaborate and practically useful series 
of such tables has been prepared by Dr. Chas. E. Emery of New 
York.1 Such an investigation often shows that the saving in fuel 
resulting from the use of a more than usually efficient, but expen- 
sive engine, is not sufficient to warrant the increase in first cost. 
Many similar but less complete investigations of the cost of steam 
power have been published.? : 
Supposing however the type of boiler and engine to have been 
selected, the proportions and size of the engine necessary for 
maximum financial efficiency can be determined with a very fair 
approximation to accuracy. Perhaps the most successful attempt 
at accomplishing this is due to Prof. Thurston,? who (following 
up a method suggested by Rankine‘) has elaborated a system by 
which can be determined the ratio of expansion involving the 
least annual expenditure. The assumption is made that the ratio 
of expansion is an independent variable upon which the various 
losses and costs depend, and from the best ratio so determined 
the proper size of engine can be at once computed. 
2. It may seem somewhat arbitrary to draw a line between the 
mechanical and the scientific side of engine design, but for pur- 
poses of convenience such a distinction will be made by consider- 
ing the mechanical side to embrace all that goes on after the 
conditions of working such as the steam pressure, ratio of expan- 
sion and speed have been determined together with the size and 
under practical conditions.”—Sib. Jou rn. Eng., March, 
e.g-, Unwin’s “ Development and Transmission of aes Chap. UT 
3 Thurston—« M. anual of Steam Engine,” Part r., Chap. vit.; also Proc: 
Inst. Naval Arch. 1895. 
4 Ship Building, appendix. 
