% Mr. Robertson on the Precession 
gravity. For if the causes be adequate to the production of the 
same velocities, taken separately, and in the same directions 
the velocity and direction of a particle will be the same from 
their combined influence upon it, whether these causes be 
impulses or constant forces. 
As the body is understood to be in free space, if the causes 
of the revolutions, taken separately, be instantaneous im- 
pulses, and made at the same time, immediately after their 
agency the body will revolve about the axis TS, and it will 
continue so to revolve with an uniform velocity. If whilst 
the body is revolving with an uniform velocity about the axis 
DC, a constant force begin to act at D, so as to cause a ten- 
dency to revolution about AB, as stated in article 5, and 
continue afterwards to act at T, the pole of the new &xis, 
from a combination of the constant agency at the new pole 
and the uniform velocity about DC, the axis TS will inces- 
santly shift its position. 
Such exactly are the circumstances to which the earth is 
subject as to the production of the precession of the equinoxes. 
At the vernal equinox, for instance, a straight line drawn 
from the centre of the sun to that of the earth is in the plane 
of the equator, and therefore, as equal portions of the pro- 
tuberant matter of the earth are above and below the 
ecliptic, the attractive power of the sun has no tendency to 
alter the position of the equator. But, in consequence of the 
earth's motion in its orbit, it very soon after the equinox 
presents a different position of the equator to the sun. The 
equilibrium of the protuberant parts of the earth, above and 
below the ecliptic, and towards the sun, is then done away. 
