THE STEAM-ENGINE. 



Fig. 13. 



through the other dotted semicircle, and would consequently continue to drive 

 the sun- wheel round in the same direction. 



This contrivance, although in the main inferior to the more simple one of 

 the crank, is not without some advantages ; among others, it gives to the sun- 

 wheel double the velocity which would be communicated by the crank ; for in 

 the crank one revolution only on the axle is produced by one revolution of the 

 crank, but in the sun-and-planet wheels, two revolutions of the sun-wheel are 

 produced by one of the planet-wheel ; thus a double velocity is obtained from 

 the same motion of the beam. This will be evident from considering that 

 when the planet-wheel is in its highest position, its lowest tooth is engaged 

 with the highest tooth of the sun-wheel ; as the planet-wheel passes from the 

 highest position, its teeth drive those of the sun-wheel before them, and when 

 it comes into the lowest position, the highest tooth of the planet-wheel is en- 

 gaged with the lowest of the sun-wheel : but then half of the sun-wheel has 

 rolled off the planet-wheel, and, therefore, the tooth which was engaged with 

 it in its highest position, must now be distant from it by half the circumference 

 of the wheel, and must, therefore, be again in the highest position ; so that 

 while the planet-wheel has been carried from the top to the bottom, the sun- 

 wheel has made a complete revolution. 



This advantage of giving an increased velocity may be obtained also by the 

 crank, by placing toothed wheels on its axle. Independently of the greater 

 expense attending the construction of the sun-and-planet wheel, its liability to 

 go out of order, and the rapid wear of the teeth, and other objections, rendered 

 it inferior to the crank, which has entirely superseded it. 



Although by these contrivances Watt succeeded in obtaining a continuous 

 circular motion from the reciprocating motion of the steam-engine, the machine 

 was still one of intermitting, instead of continuous action. The expedient of 

 a counterweight, elevated during the descending stroke, and giving back the 

 power expended on it in the interval of the returning stroke, did not satisfy 

 he fastidious mechanical taste of Watt. He soon perceived that all which he 

 proposed to accomplish by the application of two cylinders and pistons work- 

 ng alternately, could be attained with greater simplicity and effect by a single 

 cylinder, if he could devise means by which the piston might be impelled by 

 steam upward as well as downward. To accomplish this, it was only neces- 

 sary to throw the lower end of the cylinder into alternate communication with 

 the boiler, while the upper end would be put into communication with the 

 condenser. If, for example, during the descent of the piston, the upper end 

 of the cylinder communicated with the boiler, and the lower end with the 



