SCIENTIFIC SUMMAEY. 
181 
of a small planet. Extending this view to the particles supposed upon the 
nebular hypothesis to have been travelling in a sort of cosmical cloud around 
the space now occupied by the zone of asteroids, he shows how all the par- 
ticles travelling in periods nearly commensurable with the period of Jupiter 
would be so disturbed as to take up eccentric orbits and so come into collision 
with outer or inner zones. Thus the zone they had belonged to would become 
vacant. Extending these considerations to the Saturnian ring-system, as 
affected by the nearer satellites, he shows how the theory supported in Proc- 
tor’s Saturn,” that the rings consist of multitudes of small satellites travel- 
ling nearly in one plane around Saturn, would require (on his hypothesis) that 
there should be a division in the rings wherever the small satellites would 
have a period nearly commensurable with that of one of the large satellites. 
Applying this consideration to determine whether a physical cause can be 
assigned for the great division, he has detected the following very singular 
relation. The period of a satellite revolving at a distance equal to the 
inferior limit of the great division is 10 h. 52 m. 11 s., while that of a 
satellite revolving at a distance equal to the exterior limit is 11 h. 35 m. 18 s. 
Now, between these limits lie the following proportional parts of the periods of 
the four inner satellites— one- sixth of the period of Dione (10 h. 66 m. 53 s.), 
one-third of the period of Enceladus (10 h. 59 m. 22 s.), one-half of the 
period of Mimas (11 h. 18 m. 32 s.), and one-fourth of the period of Tethys 
(11 h. 19 m. 36 s.). Certainly these coincidences are very remarkable, and 
go far to establish Professor Kirkwood’s interpretation of the gaps in the 
asteroidal zone, and of the general bearing of all such facts upon Laplace’s 
nebular hypothesis. 
The Lunar Crater Linne . — This crater is beginning to be a weariness of 
the soul to astronomers. The rival views respecting the supposed volcano 
in eruption at this point of the moon’s surface have been maintained with 
equal energy and acumen by many of our best observers. But so much 
uncertainty hangs over the whole question at present that we may be per- 
mitted to look with less interest upon the discussion of opposite hypotheses, 
than we should feel if the indications of activity had been more satisfactory. 
Mr. Birt has been diligently engaged in examining the observations of Mr. 
Huggins, Captain Noble, Baron Miidler, Professor Tacchini, and others j and 
he has constructed a section of the crater, which appears satisfactorily to 
account for the phenomena which have been observed. He remarks, that 
if any changes are taking place in the surface round the orifice of the crater, 
these changes can hardly fail to be indicated by corresponding variations in 
the epochs of the disappearance and reappearance of the shadow after sun- 
rise at the crater, and before sunset. In a letter to Mr. Birt, Baron Madler 
remarks that the great whitish spot surrounding Linn^, as shown in the 
English observations and in Tacchini’s drawings, was never seen by him 
in 1831. The crater was then surrounded by the greenish colour of the 
Mare Serenitatis. 
The Planets during the next Quarter . — Jupiter will be in conjunction 
with the sun on April 16, until which time he will be an evening star, 
though daily becoming less favourably situated for observation. Saturn is 
slowly returning to our nocturnal skies, and will be very favourably situated 
for observation from the end of May. His ring-system, being fully open, will 
