MACKIE — ON SPIRAL PLANETART OEBITS. 
83 
The notion of the resistance of the ether of space to planetary 
I orbital motion, has been passing through other minds besides our 
own, and since we printed them we have read with much pleasure 
an article bj Professor Henrichs, of Iowa, on the Density, Rotation* 
and Age of the Planets, in the ' American Journal of Science.' All 
the old astronomical notions of the old astronomers of the perma- 
nence of the heavenly orbs and the celestial system, still retained, 
and too closely and superstitiously adhered to, are based upon 
and must be based upon erroneous foundations. The permanence 
and endurance of heavenly bodies, and the perpetual rectification and 
everlasting persistency of their motions, must be based upon these 
assumptions: — 1, that space is a vacuum; 2, that a body set in mo- 
tion in vacuo will continue in motion ad infinitum ; 3, that every 
form of motion must be an accurate mathematical figure perfectly 
and absolutely true, such as a circle, an ellipse, a straight line ; and 
lastly, that there must never be any transportation or intercom- 
mingling of even the merest particles of matter between one world 
and another, nor the least loss nor the least gain of material from 
any planet or sun whatever, from the beginning to the end of time. 
The world at its creation must liave been weighed in the balance, 
and not an ounce nor a feather's weight added to it since. Now we 
know there is ether in space ; if there were not, the light of the sun 
would have no material to vibrate upon, and tliough the sun burned 
with ten times its glorious brilliancy, all would be darkness here, for 
jt is scarcely possible to believe that light could travel in an absolute 
vacuum. We never get, experimentally, a perfect vacuum, pump as 
hard as we can ; fit the joints of our instrument as close as we may, 
we never get one. It is always an atmospheric vacuum, a nitrogen- 
vacuum, a hydrogen-vacuum, an ether-vacuum, — always some lesi- 
due ; never a vacuum at all, however near it may approach, — always 
some material particles, no matter how expanded, for the vibration of 
light to thrill along, and this the electric spark and the spectrum- 
prism will always show. If the ether of space exist,— and astrono- 
mers who adhere strongest to the old notions admit it, — there must 
be resistance. The more subtle the ether, the more delicately slight 
the resistance, but still resistance ; and with resistance comes fric- 
tion, with friction retardation and the evolution of heat. AVith re- 
tardation of orbital motion the diameter of the planet's orbit must 
be contracted. The world might— most probably would— run round 
the sun up to the very same line, radiating from our luminary to the 
same hour, minute, and second, but not to the same spot, but to a 
