238 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 deduced 

 from 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 para- 

 bolic 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 equiva- 

 lent 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 



