98 Astronomical and Nautical Collections. 
other planetary perturbation, that ca be imagined, enable us to 
remove this error. 
I therefore think myself entitled to consider it as demonstrated, 
that an alteration of the supposed masses of the planets alone is 
not sufficient for the purpose; and in the predictive computation of 
the places for 1822, which has now been verified, I was induced by 
this conviction to assume an empirical correction proportional to 
the square of the time. 
Such a correction of the epoch corresponds to a mean angular 
motion increasing directly as the time, and points at once to the 
possibility of an explanation of its foundation. From Newton 
down to Laplace, a number of the most acute mathematicians have 
investigated the influence which any substance, scattered through 
space, might have on the motions of the heavenly bodies. The 
resistance of such a medium would necessarily occasion a continual 
shortening of the greater axis, and consequently an increase of the 
mean motion, and a diminution of the eccentricity; while the 
longitude of the perihelium would undergo only periodical changes 
compensating each other in the time of a revolution; and the 
nodes and inclination, which affect only the plane of the orbit, 
would be perfectly exempted from its influence. Now these are 
exactly the changes which are observable in our comet; for the 
decided change of the eccentricity, in the year 1822, may at least, 
in great measure, be attributed to some extraneous cause: though 
the earlier observations would not agree so well with a rapid 
change of this element. 
Before it had been shown by these computations that the various 
returns of the comet could not be reconciled to each other without 
some such hypothesis, Dr. Olbers had already pvinted out the pro- 
bability of a similar alteration in a letter to Mr. von Lindenau. 
He wrote again to me on the subject in these words: ‘ ‘The ex- 
emption of the dense and solid bodies of the planets from any sen- 
sible effects of resistance, in the interplanetary spaces, proves 
nothing with respect to comets, which occupying, perhaps, a 
volume 1000 times as great, may have masses 1000 times as 
small: and with respect to this comet of ‘ Pons,’ such a resistance 
