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 /orce(i to de- 
part by obstructiofis — 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 joints 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 
36), are bound over to the British Islands, to the North Sea, and 
