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 be— friction ; 

 and friction, however slight, effects 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 within 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 

 orbits, 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 spiral orl)it 1 



