321 



made on other genera of Hydroida, the author maintains the presence 

 of a true medusoid structure in the fixed ovigerous vesicles of all the 

 genera he has examined, and he arrives at the generalization, that for 

 the production of true ova in the hydroid zoophytes, a particular 

 form of zooid is necessary'', in which the ordinary polype -structure 

 becomes modified, and presents, instead, a more or less obvious 

 medusoid conformation. Hydra being at present the only genus 

 which appears to offer an exception to this law, though the author 

 believes that the exception is only apparent, and that further obser- 

 vations will enable us to refer the reproductive organization of this 

 zoophyte to the same type with that of Cordylophora and the marine 

 Hydroida. The author has satisfied himself that the ova-like bodies 

 contained in the capsules of Cordylophora are true ova, and not 

 gemmcE ; he has demonstrated in them a distinct germinal vesicle, 

 and has witnessed the phenomenon of yelk- cleavage ; and the paper 

 details the development cf the embryo to the period of its escape 

 from the capsule in the form of a free-swimming ciliated animacule, 

 and traces its subsequent progress into the condition of the adult 

 zoophyte. 



2. " On the Secular Variation of the Moon's Mean Motion." By 

 J. C. Adams, Esq., M.A., F.R.S. &c. Received June 16, 1853. 



The author remarks, that in treating a great problem of approxi- 

 mation, such as that presented to us by the investigation of the 

 moon's motion, experience shows that nothing is more ea«y than to 

 neglect, on account of their apparent insignificance, considerations 

 which ultimately prove to be of the greatest importance. One in- 

 stance of this occurs with reference to the secular acceleration of the 

 moon's mean motion. Although this acceleration and the diminu- 

 tion of the eccentricity of the earth's orbit, on wdiich it depends, 

 had been made known by observation as separate facts, yet many of 

 the first geometers altogether failed to trace any connexion between 

 them, and it was not until he had made repeated attempts to explain 

 the phenomenon by other means, that Laplace himself succeeded in 

 referring it to its true "cause. 



The accurate determination of the amount of the acceleration is a 

 matter of very great importance. The effect on the moon's place, 

 of an error in any of the periodic inequalities, is always confined 

 within certain limits, and takes place alternately in opposite direc- 

 tions within very moderate intervals of time, whereas the eflfect of 

 an error in the acceleration goes on increasing for an almost inde- 

 finite period, so as to render it impossible to connect observations 

 made at very distant times. 



In the ' Mecanique Celeste,' the approximation to the value of 

 the acceleration is confined to the principal term, but in the theories 

 of Damoiseau and Plana, the developments are carried to an im- 

 mense extent, particularly in the latter, where the multiplier of the 

 change in the square of the eccentricity of the earth's orbit, which 

 occurs in the expression of the secular acceleration, is given to terms 

 of the seventh order. 



