THE FLY--VVHEEL. 29 



required to give it momentum as it afterward imparts 

 to the machine ; it consequently only accumulates and 

 regulates power. 



A curious example of the effect of momentum is 

 shown in the failure and success of two different modes 

 of constructing wire fences with very slender wires for 

 the boundaries of pastures. The unsuccessful mode 

 consisted of tightly-stretched wires between solid posts 

 not more than twelve to twenty feet apart. A side 

 strain of only a few mches was enough to snap the 

 wires ; consequently, a bullock plunging blindly against 

 them could not be quickly enough checked in his mo- 

 mentum, and such fences were therefore nearly useless 

 without stronger wire. The successful mode was to 

 stretch the wires well between strong and deeply-set 

 posts some hundreds of feet apart, the intervening space 

 being kept even by upright bars, but not posts. When 

 an animal accidentally struck this fence, the great 

 length permitted it to yield sidewise far enough to ex- 

 pend the momentum without rupture, when its elas- 

 ticity at once threw it back to its former place. 



On rough roads, the force of inertia causes a severe 

 strain to a loaded wagon when it strikes a stone. The 

 horses are chafed, the wagon and harness endangered, 

 and the load jarred from its place. This inconvenience 

 is avoided in part by placing the box upon springs, 

 which, by yielding to the blow, gradually lessen the 

 effects of the shock. For carts and slowly - moving 

 lumber-wagons their advantages are considerable, but 

 much greater as the velocity and momentum increase. 

 Even on so smooth a surface as a rail-road, it was 

 found, by experiments made some years ago, that when 



