§2 THE ZOOLOGIST. 
When we say that the force which moves the engine comes 
from the coal that is burning in the furnace, and is conducted 
through pipes by the medium of steam to a movable piston which . 
it sets in motion, we have in a general way given an explanation 
to the activities there going on. When we say that the grind- 
stone is operated by the force derived from the muscular 
organisation of the boy turning it, then its action is also 
explained briefly. 
It is in this way that an attempt will be made to explain a 
soaring bird. No objection is taken to the view that force cannot 
produce motion, held by some recent scientists. Granted that 
nothing but motion can produce motion, and then I am only con- 
cerned with the sequence of events; with having it understood 
that the motion of the piston is not the cause of the burning coal, 
nor that the grindstone turns the boy. 
When the trifling tornado struck the birds, as above related, 
the mechanical activities going on between bird and air were 
thrown into confusion, and the gravitating force of the bird’s mass 
instantly carried it to the water. It was evident that the internal 
adjustments to environing conditions, going on through a line of 
ancestry reaching to the Reptiles of the secondary age, omitted 
summer cyclones. They were too rare to count. It was also 
pretty clear that the gravity of the bird’s mass was the source of 
the entire motive power concerned in the act of soaring. 
Were we dealing with wind-mills, sailing-vessels, tornadoes, 
or any other phenomenon in which the air was one factor and a 
body connected with the earth the other, the force would properly 
be spoken of as coming from the air. The amount of force would 
vary with the velocity of the wind. The work done would be 
referred to the mechanical agency which set the air in motion. 
But a body suspended in free air is part of the atmosphere, and 
at rest with it, unless it employs some activity not derived from it. 
The same mechanical agent which moves the air equally moves the 
body. ‘The active birds derive the force to move themselves in the 
air from their muscular efforts, the soaring birds from gravity. 
Gravity gives all the motive power —that which antagonises itself 
and that which antagonises air-resistance. 
The case is analogous to that of a man on a moving train of 
ears. He is at rest with the train throughout, unless he employs 
muscular power to set up motion with it. All activity between 
