SECT, iv.] AND PROPORTIONS OF ENGINES. 163 



330. If we refer to the practice of engine makers, we find no indication of a 

 settled rule for the proportions of the cylinder, when the length of the stroke is not 

 limited by convenience. The proportions followed at different times by Boulton 

 and Watt, in cases where the stroke was not limited, vary from If to nearly 3 to 1, 

 the most common about 2'7 to 1, the changes having no regularity. In Smeaton's 

 table of the proportions of atmospheric engines, 1 the length of the stroke is made 

 to vary nearly as the square root of the diameter, and commences at the lower 

 part of the scale with the proportion of 4 to 1 ; why the square root of the 

 diameter was fixed upon does not appear. Equally irregular are Maudslay's 

 proportions, but approaching to 2 to 1 ; Fenton, Murray, and Wood's about as 2^ 

 is to 1. The object seems to have been to render the velocity nearly the same in 

 all engines : the circumstances which regulate the velocity may therefore next be 

 considered. 



OF THE MAXIMUM OF USEFUL EFFECT IN STEAM ENGINES. 



331. In steam engines there is a certain velocity for the piston, which gives a 

 maximum quantity of useful effect. 



In an engine already constructed, the velocity which gives the most useful effect 

 that the engine is capable of producing, is limited by the proportions which have 

 been given to the parts of the engine. 



But in an engine to be designed, all the parts should be arranged to agree with 

 the velocity which gives the maximum effect of a given quantity of steam : the 

 difference between these cases is considerable ; but in illustrating each by example, 

 I shall have an opportunity of showing that a general rule could not be derived 

 from experiments on a particular engine. 



OF THE MAXIMUM FOR ENGINES EQUALISED BY A FLY. 



332. Our most simple case for consideration is that where the pressure on the 

 piston is the same throughout its stroke ; and we must suppose the fly, conjointly 

 with the mass of matter in the engine, to be so proportioned as to render its 

 velocity as nearly as possible uniform. 



Then the greatest uniform velocity the engine could possibly acquire, would be 

 equal to half that which a falling body would acquire in descending the length 

 of the stroke ; and with this velocity the work done would be nothing ; as the 

 whole force of the steam would be expended in keeping the engine moving at that 

 velocity. 



1 Rees's Cyclopaedia, art. Steam Engine. 



