236 LIGHT SCIENCE FOR LEISURE HOURS. 



the shot from the barrel ; with ' slow-rifling,' these con- 

 ditions are reversed. 



And then the common notion is that a cannon-ball 

 travels in the curve called a parabola, and that artil- 

 lerists have nothing to do but to calculate all about 

 this parabola, and to deduce the range from the initial 

 velocity according to some simple principles depending 

 on the properties of the curve. All this is founded on 

 a complete misapprehension of the true difficulties in 

 the way of the problem. Only projectiles thrown with 

 small velocity from the earth travel in parabolic paths. 

 A cannon-ball follows a wholly different kind of curve. 

 The resistance of the air, which seems to most persons 

 a wholly insignificant item in the inquiry, is so enormous 

 in the case of a cannon-ball as to become by far the 

 most important difficulty in the way of the practical 

 artillerist. When a 250-lb. shot is hurled with such 

 force from a gun as to cover a range of six miles, the 

 resistance of the air is about forty times the weight of 

 the ball that is, is equivalent to a weight of upwards 

 of four tons. The range in such a case as this is but a 

 small fraction of that which would be given by the 

 ordinary parabolic theory. 



As regards artillery practice in war, there are other 

 difficulties in the attainment of a very extended range. 

 Cannon meant for battering down forts cannot possibly 

 be used in the same way that Whitworth's was used 

 at Shoeburyness. If the shot flung from this gun at 

 an elevation of thirty-three degrees could have been 

 watched, it would have been found that it fell to the 



