24 MECHANICS. 



stronger than the upper, the inertia of the weight re- 

 quiring a great force to overcome it suddenly. The 

 threads used in this experiment may be easily had of 

 any desired strength hy taking the finest sewing cot- 

 ton, and doubling to any required extent. 



This experiment shows the reason why a horse, when 

 he suddenly starts with a loaded wagon, is in danger 

 of breaking the harness ; and why a heavier weight 

 may be lifted with a windlass or pulley having a weak 

 rope, if the strain is gradual and not sudden. For the 

 same reason, glass vessels full of water are sometimes 

 broken when hastily lifted by the handle. When a 

 bullet is fired through a pane of glass, the inertia re- 

 tains the surrounding glass in its place during the mo- 

 ment the ball is passing, and a round hole only is 

 made ; while a body moving more slowly, and pressing 

 the glass for a longer space of time, fractures the whole 

 pane. 



SECTION II. 

 MOMENTUM. 



The force which a moving body has to continue on- 

 ward is called its momentum ; it is, in fact, the inertia 

 of a moving- body. When a force is first applied to 

 any heavy body, it moves slowly ; but the little mo- 

 mentum it thus acquires, added to the continued force, 

 increases the velocity. This increase of velocity is 

 of course attended with increased momentum, which 

 again, added to the acting force, still further quickens 

 the speed. For this reason, when a steam-boat leaves 

 the pier, and its paddle-wheels commence tearing 



