28 A. C. D. CROMMELIN, ESQ., D.SC., F.R.A.S., ON 
the comets of 1862, 1866, and Biela’s comet with the Perseid, 
Leonid and Andromedid meteor showers respectively, and further 
that the meteors that have fallen to earth and have been chemi- 
cally analysed have been found to contain much occluded gas, 
especially hydrogen, which with its compounds is indicated in 
cometary spectra, it seems to me a most natural and probable 
deduction to draw that the reservoir containing the gas of comets’ 
tails is a dense form of meteors ; in fact, I should scarcely have 
thought it a matter of dispute, had not several well-known 
astronomers expressed doubts about the connection of comets 
and meteors. There is the further argument for the presence of 
a nucleus made of solid matter, that it appears to move exactly 
as if under the force of gravitation alone. 
The calculations of its motion are made on this assumption, 
and the difference between theory and observation in the time 
of its perihelion passage amounts to only three days in a period 
of some 27,000 days, showing that the action of non-gravitational 
forces on the head is barely sensible; but on the tail matter 
these repulsive forces far exceed gravitation, showing that the 
particles of the nucleus are much denser than those of the tail, 
and no doubt solid. I even venture to assert that the solid 
matter in the head of Halley’s comet is not mere dust, but is in 
the form of pretty large lumps, at least several feet across, 
since otherwise I should expect the supply of gas to have been 
exhausted after a few returns. I think it is likely that the loss 
of gas occurs only when the comet is near the sun, the occluded 
gas being drawn out, either by the action of heat or some other 
exciting cause. When in the cold of outer space it probably 
sinks into a torpid condition and is devoid of envelopes. 
An exceedingly rare event is about to happen this month, 
which may throw some light on the constitution of the comet’s 
head; [I make out that this event, the transit of the comet over 
the dise of the sun, only happens if the perihelion passage falls 
in one particular half day of the entire year; that is, that 
one return in 700 or once in 50,000 years. Unfortunately 
the sun will be below our horizon when the comet crosses it, 
but astronomers in more favoured lands will be on the alert, 
notably at the Kodaikanal Observatory in India, whence Mr. 
Evershed writes to me that they are making preparations to 
photograph the sun in ultra-violet light, and in other methods 
that seem to give the best hope of success. 
Let us however consider the conditions, and we shall see that 
failure is quite likely; the comet will be 15,000,000 miles 
distant, or sixty times as far away as the moon. At that 
