16 Examination of the Theory of a Resisting Medium. 
in. the observed duration of its period, due to this resistance. Valz, 
from the computations of Damoiseau, finds this diminution to be 
eight tenths. Prof. Santini, frorm his own elements finds four tenths, 
while Encke’s formula and constant, for computing this acceleration, 
only accounts for a diminution of three hundredths of a day. ‘The 
mean of the three results would show that Biela’s comet experiences 
the resistance of a medium twenty-five times as powerful as that 
which is encountered by Encke’s comet.”(55) Halley’s comet re- 
mains to be noticed. We have seen that the two above were 
accelerated, though very unequally, the cause of which was sup- 
posed to be the resisting ether. . But .Halley’s comet, in its return 
to its perihelion, in 1835, was, from some cause, detained beyond its 
time for arriving at that point—a result directly opposite to that in 
the case of the other two bodies. <‘‘In traversing a resisting ether 
the comet of Halley would have arrived at its perihelion, in 1835, 
sooner than if moving in a void; now on the contrary, according to 
the calculations of M. Rosenberg, that body, by observation, was 
stx days behind its time, according to the results of calculations dis- 
connected from any allowance for the action of resisting ether. The 
difference, though much less, found by M. Pontecoulant, is of the 
same kind! Hitherto, then, the last appearance of Halley’s comet 
has added nothing to our knavlexe of the poy constitution of the 
celestial spaces.” (56) 
We have said the acceleration of these bodies is unaccounted 
for: so is the retardation; but we shall presently see whether other 
agents than ether may, within the bounds of probability, be supposed 
to give rise to these. Clairaut, in announcing to the French Acad- 
emy, in 1758, that the then expected return of Halley’s comet would 
be retarded six hundred and eighteen days beyond its previous pe- 
riod, by the combined action of Jupiter and Saturn, adds that, “a 
body which traverses regions so elongated from the sun, and which 
escapes, for so long periods, from our view, may be subject to forces 
totally unknown; such as the action of other comets, or even of 
planets, so distant from the sun as to have remained hitherto un- 
discovered.”’(57) Uranus was unknown until 1781, twenty three 
years after this announcement; and four other planets, belonging to 
our system, have been discovered within the present century—in all 
——— 
(55) S.C. Walker, Preface to Herschel’s Astron, | 
(56) Arago, Annuaire, pour l’an 1836. 
(57) Laplace, Systeme du Monde, p. 214. 
