SCIENTIFIC SUMMARY. 
93 
by the mutual attraction of Pales, Diana, and Parthenope P This, to myself, 
seems wholly improbable. The required explanation is to be looked for in 
the perturbing influence of Jupiter and Saturn. A comparison of mean 
motions gives the following equations : — 
2n( n ) — 9nv + 7n vi = 0 .... (3) 
n ( 5 °) — 3 n y f 2n = 0 . . . . (4) 
w (78)_4 w v + 3 w vi = 0 . . . . (5) 
Eliminating^ and from (3), (4), and (5), we obtain equation (1). 
Other results have been reached, which will probably soon be published. 
The Zodiacal Light. — Mr. Plummer, having been engaged observing 
Comet III. 1874 (Coggia’s) upon several occasions during the early morning 
hours in the present and preceding months, has noticed repeated brilliant 
exhibitions of the zodiacal light. Upon seven mornings it has been very 
conspicuous indeed, more than rivalling the brightness of the Milky Way, 
namely, on September 17, 19, 21, 22, October 9, 19, and 21, at hours 
varying from 14h. 15m. in the former month, to 16h. Cm. in the latter. On 
October 10 it was invisible only in consequence of a general phosphorescent 
state of the sky. “ As these were the only dates,” he says, “ upon which I 
had the opportunity of observing the phenomenon, it appears to justify the 
conclusion that it has been much more distinct and brilliant this autumn 
than is usually the case. This confirms a precisely similar opinion which 
was forced upon me during the spring of the present year, and which I had 
already communicated to several astronomical friends. It is the more im- 
portant, as a like period of maximum brilliancy appeared from my own 
observations to have taken place about the spring of the year 1866 ; which, 
if correct, would seem to indicate a periodical variation of brightness that 
has not been previously noted. I am induced to call attention to the 
matter, as it is not unlikely that the ensuing spring may also be a favour- 
able season for observing it. I may mention that I have long watched with 
interest for the annual manifestations of the zodiacal light ; and although 
my remarks thereon are scattered through the notebooks of different 
observatories, the above suspicion, which is founded upon them, may lead 
to more attention being paid to it than formerly. If Southern localities are 
more advantageous for the spectroscopic examination of the zodiacal light, 
there seems no reason why variations of brilliancy, supposing such to pccur, 
may not be observed in England. Indeed, after making allowance for the 
want of purity of our atmosphere, and to some extent also for the greater 
duration of twilight, there would seem but little reason why the zodiacal 
light may not be as well observed here as in subtropical regions. As Colonel 
Tomline’s observatory will be furnished with spectroscopic appliances 
before the next apparition of the zodiacal light, I shall not then fail to 
give the subject careful consideration in each of these aspects.” 
