All About Engines

The Modern Reciprocating Engine 113 slide about the floor. Should the holding-down bolts give way, it would become a very lively companion indeed for the man in charge. Now the most important fact to be observed is that this force varies as the square of the velocity ; that is, if the velocity is doubled the force is quad- rupled ; if the velocity is halved the force is reduced to one-quarter. Thus, in the engine in question, if the speed were reduced to 100 revolutions per minute, the force would be re- duced to about 340 lb. On the other hand, at 300 revolutions per minute the force would become about 3,200 lb., or nearly a ton and a half. An unbalanced en- gine running at high M.-Maneed o™k speed threatens, therefore, to tear itself to pieces. So long as engines were slow moving, these considerations were not important, but the tendency towards high speeds during the last thirty years has introduced problems of great difficulty and delicacy into the work of engine building. It is useless to put pounds’ worth of material and months of labour into a machine that will rapidly accomplish its own de- struction. Fortunately, there are methods of obtaining an approximate balance which are neither very costly nor very difficult to adopt. Probably most readers