MACKIE — ON PLANETARY ORBITS. 
443 
Now, as the earth possesses no apparatus for, or power of producing 
its own onward motion, we have no alternative but to consider the 
orbital motion of the earth as due to an initial velocity imparted to 
it originally at some vastly distant period of time. But if we regard 
the earth's motion as originally initial, it becomes at once certain 
that if the earth in its orbit pass through the ether or matter with 
which the circumambient space around us is filled, that there must be 
— no matter how fine, how attenuated that ether may hQ— friction ; 
and friction, however slight, eff'ects retardation. Moreover, the moon 
retards the earth : as Dr. Tyndall has well expressed it, " she skids 
the earth." Now, if there be a gradual retardation of the earth in 
her orbit, there is a gradual diminishing of the centrifugal velocity 
which counteracts the sun's attraction ; -and if the centrifugal com- 
pensation diminish, the earth must gradually be approaching the sun ; 
for the earth's orbit, if originally circular or elliptic, must assume a 
spiral condition directly there is any alteration of the great counter- 
balance of the sun's attraction by centrifugal force, whether that 
alteration be increased or diminished attraction, or increased or 
diminished velocity of revolution ; for with an inward tendency 
towards the sun the spiral would be a gradually approaching one, and 
with outward tendency to fly off" into space, a gradually extending 
one. Nor would this spiral orbit produce any effect on the sidereal 
day or year, for the loss of velocity in the earth would be counter- 
acted exactly by the inward contraction of the spiral and the conse- 
quent shorter distance to run. Again, if the earth's orbital motion 
be due to an initial velocity imparted in the remote past, the earth's 
orbit must still be a spiral one, because, if initial and imparted, 
the original velocity had the nature of projectile force, and the ten- 
dency of the original motion must have been in a direct or straight 
line until the earth came ivithin range of the sun's attraction ; and 
this attraction diminishing, as a perfect sphere enveloping the central 
attracting globe or sun, must have been, as far as its influence was 
exerted upon the projected world, a circular influence, from which 
the earth could not extricate itself, and the resulting figure of the 
combination of a straight line and a circle must necessarily be a 
spiral. 
For very many years I have held this idea of spiral planetary 
orlits, and the recent letter of Mr. Hind, the astronomer, on the 
possibility of the variation of our earth's distance from the sun, 
seems to give support to such a conclusion. Certainly a sjnral orhit, 
