VIII
A:a = M:m
M1:m1, M. M1.d2:m.m1.D2
d2: D2
=M.M1/D2:m.m1/d2
§ 22. Bearing in mind the magnitude of the earth's radius
(4000 miles), all bodies on the surface of the earth may be con
sidered as being at the same distance from the earth's centre;
thus the difference of the manner, in which they are attracted
is only due to their difference in weight. Gravity reveals itself
in the pressure of one body on another, in traction (drawing)
and in falling (see Chapter III).
§ 23. The pressure of a body upon its support is called its
Weight. We make a difference between Absolute Weight (the
proportion, in which the pressure of a body stands to the pres
sure of certain fixed bodies, called weights) and specific weight
(in which the absolute weights of two bodies of equal volume
are compared, the unit being water; see 167).
§ 24. On account of the earth's flattening at the poles and
on account of the decrease of the centrifugal force (III, 3) from
the equator to the poles (which force impedes gravity), the weight
of a body increases from the equator to the poles.
§ 25. A line drawn in the direction, in which gravity acts
is called a vertical line, and a line perpendicular to this line is
a horizontal line. Owing to the tractive action of gravity by
means of the Plummet (Plumb-line) people may know, whether
a wall is vertical or not.
Remark. It was Newton who, seeing an apple fall from a tree in his
garden, first gave clear expression to the law of gravitation.
§ 26. 11) Centre of Gravity. 95-108. The point, through
which gravity acts, or the point, in which gravity may be neu
