THE STEAM-ENGINE. 159 



Obviously, the non-expansive engine has a tendency to 

 produce variation of motion, that is to say, the tangential 

 force will be zero when the crank is on the centre, and it will 

 be at the maximum when the crank is at half-stroke. Jn 

 constructing these diagrams I assumed the connecting rods 

 to be of infinite lengths, and we need not therefore take into 

 account the variation that would arise from the effect of tho 

 angle made with tho connecting rod. By the side of this we 

 see an engine cutting off at half-stroke, and you will see now 

 by Fig. 2 below it that the variation in tangential force is 

 very considerably increased ; Fig. 3 shows an engine cutting 

 off at one-eighth stroke, and you will see below the increase 

 in inequality of the tangential force. Fig. 4 shows a diagram 

 from a high and a low-pressure cylinder having the respective 

 cubic capacities of one and four, and wherein the steam is 

 cut off at one-half the stroke in the high-pressure cylinder, 

 so that it is expanded eight times, as in the case of the 

 single cylinder engine considered in Fig. 3. 



Fig. 4 shows the aggregate effective pressures of the two 

 pistons at various points of the stroke, and below this the 

 variation in tangential results obtained from these cylinders. 

 There is no doubt whatever that by using two cylinders, 

 we have been able very much to reduce the irregularity 

 in the tangential force which existed with a single cylinder 

 when cutting off at one-eighth stroke. I will also call 

 your attention to another Diagram (11) showing the benefits 

 arising from having more cylinders than one with cranks 

 placed at different angles, so that with two cylinders when 

 one is on the dead point the other is doing its fullest duty. 

 An inspection of this Diagram 11 will show that with a pair 

 of cranks at right angles, and with two single cylinders 

 cutting off at half-stroke, there is a great approximation 

 to uniformity of tangential force throughout the whole revo- 

 lution. 



Let us now consider whether it is worth while to go to the 

 complication of a double cylinder engine for the purpose of 

 obtaining this greater uniformity of tangential force. Nobody 

 objects to the use of a single cylinder non-expansive engine. 

 One is content with that, and no one uses the word irregu- 

 larity. I . should now like to ask you to bear with me for 

 a short time while I go into the question of the fly-wheel. 

 It is true that the fly-wheel cannot give absolute regularity 







