Astronomical and Nautical Collections. 99 
seems to be almost demonstrable @ priori: for it moves, during a 
considerable portion of its period, in that part of the open space of 
the system, in which the visible substance of the zodiacal light, or 
solar atmosphere, is found. It is this same comet, through the 
middle of which Herschel, on the 9th Nov. 1795, saw a small 
double star of the twelfth or thirteenth magnitude, with very little 
diminution of its brightness. This fact seems to demonstrate that 
the density of the comet bears some finite proportion to that of the 
zodiacal light, and that the substance causing this light may afford 
some sensible resistance to the motion of the comet. If then all 
the rest of the space surrounding us were to be considered as per- 
fectly void and free from resistance, which I do not believe, still 
the substance of the zodiacal light, which does certainly exist, is 
sufficient to explain the phenomena of a diminution of the periodi- 
cal time, and of the eccentricity of the orbit.” 
Dr. Olbers further remarks that this ethereal medium may na- 
turally be supposed to revolve with the planets from west to east, 
so as to exhibit little or no influence on their motions, while those 
of the comets, being more discordant, may suffer a very material 
disturbance from its resistance. 
[Upon a probable supposition respecting the law of the density 
of the resisting medium, Professor Encke proceeds to calculate the 
places of the comet for 1795, 1805, and 1819, in such a manner as 
to reduce the sum of the errors to less than half their amount upon 
the most advantageous suppositions respecting the planetary per- 
turbations. For 1822 the improvement does not appear to be so 
considerable. ] 
The times of the successive passages through the perihelium, as 
affected by the perturbation, are assigned in this table, and those, 
which have been observed, are distinguished by an asterisc. 
*1786 Jan. 30.1 Parisian M.'T. *1805 Noy. 21.5 
1789 May 18.7 1809 March 11.9 
1792 Sept. 4.1 1812 June 26.3 
*1795 Dec. 21.4 1815 Oct. 12.7 
1799 April 11.1 *1819 Jan. 27.3 
1802 Aug. 1.9 *1822 May 24.0 
