THE STEAM-ENGINE. 



cylinders work together, it follows, that a quantity of steam sufficient to fill 

 four cylinders supplied by the boiler to the engine will move the train through 

 a distance equal to the circumference of the driving wheels ; and in accom- 

 plishing this, each piston must move twice from end to end of the cylinder ; 

 each cylinder must be twice filled with steam from the boiler ; and that 

 steam must be twice discharged from the cylinder through the blast-pipe into 

 the chimney. 



If the driving-wheels be five feet in diameter, their circumference will be 

 fifteen feet seven inches. To drive a train with a velocity of thirty miles an 

 hour, it will be necessary that the engine should be propelled through a space 

 of forty-five feet per second. To accomplish this with five-feet wheels, they 

 must be therefore made to revolve at the rate of very nearly three revolutions 

 per second ; and as each revolution requires two motions of the piston in the 

 cylinder, it follows that each piston must move three times forward and three 

 times backward in the cylinder in a second ; that steam must be admitted six 

 times per second from the steam-chest to each cylinder, and discharged six 

 times per second frOm each cylinder into the blast-pipe. The motion, there- 

 fore, of each piston, supposing it to be uniform, must divide a second into six 

 equal parts, and the puffs of the blast-pipe in the chimney must divide a sec- 

 ond into twelve equal parts. The motion of the slides and other reciprocating 

 parts of the machinery must consequently correspond. 



This motion of the reciprocating parts of the machinery being found to be 

 injurious to it, and to produce very rapid wear, attempts have been made to 

 remedy the defect, and to obtain greater speed with an equal or diminished 

 rate of motion of the piston, by the adoption of driving-wheels of greater di- 

 ameter, and on several of the great lines of railway the magnitude of the wheels 

 for the passenger-engines have been increased to five feet and a half and six 

 feet in diameter ; but such engines have not been sufficiently long in use to 

 afford grounds for forming a practical estimate of their effects. Experiments 

 of a much bolder description have, however, been tried on one of the great 

 lines of railway by the adoption of driving-wheels of much greater diameter. 

 In some cases their magnitude has been increased even to ten feet ; but from 

 various experiments to which these engines have been submitted by myself 

 and others, as well as from the experience which appears to be obtained from 

 the results of their ordinary work, it does not appear that any advantages have 

 attended them, and they have been accordingly for the most part abandoned. 



The pressure of steam in the boiler is limited by two safety-valves, repre- 

 sented in fig. 67, at N and O. The valve at N is under the control of the en- 

 gineer, but the valve at O is inaccessible to him. The structure of the safety- 

 valve represented at N is exhibited on a larger scale in fig. 82, which repre- 

 sents its section, and fig. 83, which shows a plan of the valve-seat with the 



Fig- 



82. 



Fig. 83. 



valve removed. The valve A, which is made of brass, is mitred round the 

 edge at an angle of forty-five degrees, and has a spindle, or stalk B, cast upon 

 it, projecting downward from the middle of it. The valve-seat C is also made 



