THE GULF STREAM. 41 



48. But there is diurnal rotation ; the earth does revolve on its 

 axis ; and since Bernini is nearer than Ireland is to the equator, 

 the gun would be moving in diurnal rotation faster than the target, 

 and therefore the marksman, taking aim point blank at his target, 

 would miss. He would find, on examination, that he had shot 

 ahead of his mark. In other words, that the path actually de- 

 scribed by the ball would not be an arc of a great circle, and that 

 the highest parallel reached by the ball in its flight would not be 

 as far north as the highest parallel touched by the great circle, 

 and that, consequently, the path of the ball would take a due east 

 course before the track of the great circle would. 



It is the case of the passenger in the rail-road car throwing an 

 apple, as the train sweeps by, to a boy standing by the wayside. 

 If he throw straight at the boy, he will miss, for the apple, par- 

 taking of the motion of the cars, will go ahead of the boy, and for 

 the very reason that the shot will pass in advance of the target, 

 for both the marksman and the passenger are going faster than 

 the object at which they aim. 



49. Hence we may assume it as a law, that the natural tenden- 

 cy of all currents in the sea, like the natural tendency of all pro- 

 jectiles through the air, is to describe their curves of flight in the 

 planes of great circles, departing therefrom — unless ybrce<i to de- 

 part by obstructioHs — only so much as the forces of diurnal rota- 

 tion may impel. 



50. The arc of a great circle is the shortest distance between 

 any two points on the surface of a sphere. Light, heat, and elec- 

 tricity, running water, and all substances, whether ponderable or 

 imponderable, seek, when in motion, to pass from point to point 

 by the shortest lines practicable. Electricity may be turned aside 

 from its course, and so may the cannon-ball or running water ; 

 but remove every obstruction, and leave the current or the shot 

 free to continue on in the direction of the first impulse, or to turn 

 aside of its own volition, so to speak, and straight it will go, and 

 continue to go — if on a plane, in a straight line ; if on a sphere, 

 in the arc of a great circle — thus showing that it has no volition 

 except to obey impulse, and the physical requirements to take the 

 shortest way to its point of destination. 



The waters of the Gulf Stream, as they escape from the Gulf 

 {k 36), are bound over to the British Islands, to the North Sea, and 



