oe find ourselves in a sort of dilemma. We are obliged to con- T 
_ Sider 
oe kno 
Le onclusion that the quantity of matter was acting, and that oy 
1886. } Gravitation and the Soaring Birds, ` | 515 
the air, which are abandoned to its support. Weighing more 
than the air they displace, and using no muscular exertion to 
sustain themselves, they still, in the sense of getting nearer to the 
earth, do not fall. When we seek for a motive power which is 
competent to resist the weight of the bird’s body, and neutralize 
the resistance of the air to its translation, we seem baffled. Bird 
and air form a material system in which no other object is in- 
cluded, so that it is impossible to obtain power from the wind. 
Wind is motion of the entire system as a whole, compared toa 
fixed object, as an observer, and such motion does not affect the 
motion of the parts. Wind from any direction, or at any velocity, 
or entire calm are differences of air-conditions to an observer, but 
not toa bird. We are therefore limited to the gravitating force 
of the bird’s body to find the power producing the phenomena, as 
it is nowhere else discoverable. But here we are confined to 
certain notions derived from sticks and stones, and in fact all 
other falling things, and they do not seem to help us in explain- 
ing a thing which does not fall. We are likewise taught that the 
direction of the gravitating force is vertically downwards, 7, e., in 
a straight line from the body manifesting it to the center of the 
earth. What we understand by this “ direction” is, that when — 
gravity does work, when it is in the act of making anything dif- 
ferent from what it was before, when it moves a thing at rest, or 
Stops it when in motion, or accelerates it, or is in the act of man- 
esting energy in the way which we call “ work,” that the “ direc- zo 
tion” in which it does it is vertically downwards. How then can >- 
this force drive a body upwards, or translate it horizontally ? : 
Still further. Although we admit, when our attention is called 
to it, that weight is the result of gravity acting on a quantity of — 
Matter, we are apt to confound mass, and weight, and gravity into — 
wog identical thing. This is inadmissible, since the doctrine of 
: € correlation of forces is established, as it is entirely possible to. _ í 
change every atom of gravitating force which a body manifests _ 
into some other form of force, in which case either the former is- —— 
Separate from the quantity of matter or the latter is created. We. = 
§ravity as something apart from body, and still we have no 
i wledge of it excepting what we are enabled to infer about it. 5 
fre it not for these inferences we would be shut up to tbe, 
