504 



axis parallel to the mean axis. If the axle of the top be the mean 

 axis, the path is an hyperbola as projected on the disc. 



When the axle is the axis of greatest inertia, the direction of mo- 

 tion in the ellipse is the same as the direction of rotation. When 

 it is the axis of least inertia these directions are opposite. All these 

 results may be deduced from Poinsot's theory, and verified by means 

 of the coloured disc. 



The theory of precession may be illustrated by this top in the 

 way pointed out by Mr Elliot, by bringing the centre of gravity to 

 a point a little below or above the point of support. 



The theory and experiments with the top suggest the question — 

 Does the earth revolve accurately about a principal axis ? If not, 

 then a change of the position of the axis will take place, not in space, 

 but with respect to the earth, so that the apparent positions of stars 

 with respect to the pole will remain the same, but the latitude of 

 every place will undergo a periodic variation, whose period is about 

 ' 325 days. To detect this variation, the observations of Polaris with 

 the Greenwich transit circle for four years have been examined. 

 There appeared some doubtful indications of a variation not exceed- 

 ing half a second. A more extensive investigation would be re- 

 quired to determine accurately the period, and the epoch of maxi- 

 mum latitude at a given observatory, which must depend on the 

 longitude of the station, as the pole of the *' invariable" axis travels 

 round the mean axis from west to east. 



3. On the true Signification of certain Reproductive Pheno- 

 mena in the Polyzoa. By Dr Allman. 



When the reproductive phenomena of Alcyonella, as manifested 

 both in gemmation and true generation, are viewed in their proper 

 sequence, they will be found to present a series of acts which admit 

 of an obvious comparison with the class of phenomena commonly 

 known as the "alternation of generations." 



From the fecundated ovum an embryo is produced in the ordinary 

 way after the segmentation of the vitellus. In this embryo, which 

 presents at first the form of a locomotive ciliated sac, sexual organs 

 are never directly developed, but there are produced within it by a 

 process of gemmation the following series of zooids. 1. A poly- 

 pide, which, like the containing sac, is essentially nonsexual, and 

 .which is eminently organized for the functions of digestion. 2. A 



