158 Mr Galbraith on Trigonometrical 



It is difficult to form a satisfactory theory of the peculiar na- 

 ture of the metamorphosis of either ascidia. Probably the body 

 of the spinula is occupied by the transparent matter, which on 

 expulsion diffuses around the front, next the radicles may be a 

 prolongation of the external skin amidst it, somewhat after the 

 nature of gemmination, and thus they form together the basis of 

 the nascent ascidia. 



From the J)receding observations, it may be deduced, Ist^ that 

 two different modes of propagation carry on the race of actinia, 

 one whereby the embryo, a shapeless corpusculum, endowed 

 with locomotion within the parent, is produced symmetrical by 

 the mouth, but then deprived of that faculty or nearly so. The 

 other whereby a fragment buds externally from the base, thus 

 generating after the fashion of the hydra tuba. 



9.d, That the Aplidium verrucosum, a compound ascidia, is 

 originally an inert ovum, next an embryo endowed with an ac- 

 tive locomotive faculty, which in the third stage is converted to 

 an animal of a form absolutely different, rivetted to the same spot. 



3cZ, That the zoophytes of certain genera pass through inter- 

 mediate stages towards perfection, of which that stage exhibiting 

 them endowed with the faculty of locomotion is not the first. 



All the preceding names are given provisionally. 



Fig. 1 Fig. 2. Fig. 3. 



Fig. 1, Spinula of the Ascidia papilla enlarged. 

 Fig. 2, Diffusing base viewed from below, enlarged. 

 Fig. 3, Ascidia Papilla^ adult, natural size. 



On Geodetical Surveying and Trigonometrical Levelling. By 

 William Galbraith, Esq. M. A. ; M. S. A,, Teacher of 

 Mathematics in Edinburgh. Communicated by the Society 

 of Arts. 



The method of surveying and levelling trigonometrically, is 

 now almost universally employed where accuracy is required, 

 and various attempts have been made to simplify the more usual 

 calculations. In consequence of being frequently employed in 



