MECHANICAL PARADOXES. 



distance at which it is possible to perform the 

 experiment in safety, it must be aimed at 

 least as high as the top of his helmet, in order 

 to allow for the inevitable drop during its 

 flight. Rifles and guns have to be sighted for 

 a point considerably higher than that which 

 they are intended to hit, so that, as in Fig. n, 

 it is possible for a bullet fired from A to hit a 

 man at B who is out of the rifleman's sight, 

 the ball passing over the roof of an intervening 

 cottage, in the direction of the curved line 



A 



FIG. 11. A PROJECTILE DEFLECTED FROM ITS COURSE. 



from A to B. But it was fired in the direction 

 of the dotted straight line from A to C, so that 

 it may be said to have turned a corner down- 

 wards. The reason why it did this instead of 

 continuing its flight in the direction in which 

 it started, is that there was a force constantly 

 acting upon it to pull it downwards namely, 

 the force of gravity, measured by its weight. 



If gravity could act horizontally the bullet 

 could be made to turn a corner sideways 

 that is, in the ordinary sense. And though 

 gravity cannot act in this way, there are other 

 forces which can. The wind, for instance, 

 blowing sideways across the firing range, acts 



42 



