(272.) 
Theory of 
a resisting 
medium not 
altogether 
favourably 
received, 
(273.) 
and still 
questioned, 
(274.) 
(275.) 
M. Encke 
on pertur- 
bations. 
858 
more nearly to the orbit of Mercury than the aphelion 
does to that of Jupiter; consequently at times the 
perturbations due to the former planet may be very 
great, and though the gravitating mass of the Comet 
is utterly unknown, yet since the momentary direc- 
tion of its motion depends solely on the ratio of the 
attractive force of the Sun and Mercury, its observed 
course gives the means of estimating that ratio,} 
The theory of a resisting medium was, on the whole, 
well received, especially in England, where some of 
our first authorities gave it their adhesion. The then 
recent establishment of the Undulatory Theory of 
Light, was thought by many to receive a confirmation 
from this evidence of something material filling the 
planetary spaces, In Germany the hypothesis of re- 
sistance received the complete opposition of Bessel’s 
high authority ; who declared that ‘a hundred other 
reasons” might be found for the fact of the accelera- 
tion, which he admits to be true. Encke, in reply, 
reduces these 100 possible hypotheses to four, of which 
we shall mention only one, as seemingly important, 
namely, the forces exerted with so much intensity 
within the body of the Comet itself, as indicated by 
the projection of the tail. But he observes, with great 
sagacity, that these forces, being apparently usually 
excited in the line of the radius-vector joining the 
Comet and the Sun, can hardly be supposed to affect 
the periodic time. It having also been objected that 
Halley’s Comet shows no trace of acceleration, but, if 
anything, of the reyerse, M. Encke truly says, that 
its perihelion distance does not lie within the assumed 
limits of the denser ether. 
Nevertheless, the theory of a resisting medium in 
space is not perhaps very popular, except in England, 
Although M. de Humboldt appears to favour it, I 
understand that the German astronomers in general 
scarcely regard it as in any degree proved. 
Yet, if not true, the cardinal fact remains unex- 
plained, The anomalous phenomena of the Tails of 
Comets, considered by Herschel to be altogether inex- 
plicable by the law of gravity, demand the closest 
scrutiny ; and one can hardly help supposing that 
the two difficulties may be in close connection, As 
the Newtonian law is now considered (since the dis- 
covery of Neptune, and the latest corrections of the 
Lunar Tables) to be absolutely suficient to account 
for everything connected with planetary motion, the 
Astronomy of Comets will be looked to with increas- 
ing interest, as likely to reveal some laws of nature 
not otherwise to be detected, In this respect, Pro- 
fessor Encke’s labours are likely to be more and more 
important in their results, 
With reference to this very eminent astronomer, 
we have only to add, that he has for a great many 
years been at the head of the Observatory at Berlin, 
and in that capacity has published an Astronomical 
Ephemeris of first-rate excellence, It is as a phy- 
MATHEMATICAL AND PHYSICAL SCIENCE. 
[Diss. VI. 
sical astronomer, however, that he will be principally 
remembered, Besides his admirable investigations 
connected with the Comet, he improved the theory of 
Vesta, and has very lately published a new Method 
of Computing Perturbations, especially for orbits con~ 
siderably elliptical, Neptune was discovered at his 
Observatory, by the assistant astronomer, M. Galle. 
Gambart’s and Biela’s Comet.—Jean GAMBART, 
one of the most promising astronomers in France, 
of consumption at a comparatively early age, I believe 
in 1836. He was director of the Observatory at Mar- 
seilles, which, notwithstanding its very unfavourable 
position in the midst of the town, has acquired con- 
siderable celebrity as regards the discovery and obser- 
yation of Comets, Pons, by whom Encke’s Comet was 
found, both in 1805 and 1818, conducted the Ob- 
servatory ; but its mounting was as bad as its situa- 
tion, and Pons used despairingly to describe his tele- 
scope as rather paralytic than parallactic, To this 
crippled establishment M. Gambart succeeded, and by 
his skill in managing his defectiye instruments, and 
by his patience in sweeping for Comets, he discovered 
and subsequently computed the orbits of a number of 
these bodies between 1822 and the period of his death. 
Gambart was highly esteemed, both by French and 
foreign Astronomers. Pons also deserves great credit 
for his extraordinary diligence in the discovery of 
Comets, and M, Valz, who still directs the Obser- 
yatory of Marseilles has cultivated this and other 
branches of the science with success, 
Gambart’s most remarkable discovery was the pe- 
riodicity of the first Comet of 1826, having detected Pesindicity 
that body independently at Marseilles, though it had i 62 years. 
been observed some days previously in Bohemia, by 
Biela, an officer in the Austrian service, It is most 
usually called Biela’s Comet, though it might with 
equal right be termed Gambart’s, who assigned its 
path and predicted its return, Clausen, about the 
same time with Gambart, assigned it a period of about 
7 years; and it was identified with former appear- 
ances in 1772 and 1805-6, Its period thus appeared 
to be 2460 days, or 63 years ; its aphelion is a little 
exterior to Jupiter’s orbit, and its perihelion is not 
much within the Earth’s, This Comet’s orbit very 
nearly intersects in one place the orbit of the Earth, 
so that had the earth been one month forwarder in 
its annual course in 1832, a collision would have 
taken place, or at least the Earth would have been 
enveloped in a cometary haze; for it is difficult to 
imagine a collision with a body whose tenuity is so 
excessive, that Sir John Herschel perceived through 
its entire thickness (estimated at 50,000 miles) stars 
of the most excessive minuteness (16th or 17th mag- 
nitude) as seen by his 20-feet reflector. It is an in- 
teresting circumstance, that the first predicted peri- 
helion passage, in 1832, took place within some hours 
of the time fixed by MM, Santini and Damoiseau, 
1 On the Masses and Densities of the Planets, see Encke in Astron. Nachrichten, yol. xix., col. 187, 
died Gambart’s 
