»8 ESSAYS anp OBSERVATIONS 
tion of the ground that makes the boat re- 
cede; it is the force of the man that is the 
only moving power, acting equally upon both 
the boat and the ground. When a boat is’ 
rowed with oars, the oars are faid to act up- 
on the water in one direction, and the water 
to react upon the oars in the opofite directi- 
on, and to produce the motion of the boat. 
Yet it is plain, that the motion is intirely pro- 
duced by the rower, who, in fo far as he 
contributes to the motion of the boat, em- 
ploys the oar as a /ever of the fecond kind, 
where the body to be moved, or the boat, 
is placed betwixt the moving power and the 
Julcrum. 
In the common account of the progref- 
fion of animals, whether it be of walking, 
flying, or fwimming, Bore/li and all other 
authors content themfelves with telling us, 
That the animal aéts upon the ground, air, 
or water, in one direction, and that the rea- 
Gtion of the medium carries the animal for- 
wards in an oppofite direGtion. This is at 
beft but too concife, not to call it a falfe the- 
ory: feeing it muft be manifeft that the me- 
dium, by any reaction which takes place _ 
here, can only confume as much force as is 
impreffed 
