1876.] On the Variations of the Earth's Magnetism, 231 



calcareous segments ; b, axial cord ; c c, cceliac canal, extending to near the 

 extremity of the pinnule ; ov, ovary filling the dilatation of the genital canal, 

 and developed from the peduncle p given off from the genital rachis of the 

 arm ; stc, subtentacular canal ; tc, true tentacular canal, with its orifices, 

 o o, leading to the tentacular apparatus. 



Fig. 10. Vertical Section of a very young Pentacrinoid, showing a portion of the Ali- 

 mentary canal al, hanging freely in the large perivisceral space jpv, and 

 the formation of the ring-canal re from which the tentacles proceed, by the 

 marking-off of the portion of the space included in the fold of the oral disk 

 or, by transverse threads of connective tissue. At an is seen the caccal ter- 

 mination of the intestine, not yet bending upwards to protrude as a vent. 



Fig. 11. Vertical Section of a somewhat more advanced Pentacrinoid, in which the 

 development of the Arms has not yet commenced, showing the enlargement 

 of the Alimentary canal al divided transversely at al', and the relative 

 diminution of the perivisceral space ; the crinoidal axis of the stem cr is 

 seen to be prolonged upwards at ax, through the central space left within the 

 intestinal coil, and to pass obliquely along the oesophageal portion ces of the 

 canal towards the oral ring-canal, re. 



Fig. 12. Vertical Section of a Pentacrinoid in which the Arms and Dorsal Cirrhi have 

 appeared : — the alimentary canal, of which ces is the oesophageal portion, and 

 hit the intestinal portion, now fills the perivisceral cavity more completely; 

 and a membranous fold mf shows itself, which divides that cavity into a 

 dorsal and a ventral portion. From the former proceeds the cceliac canal 

 cc ; from the latter the subtentacular canal stc ; while the true tentacular 

 canal tc proceeds from the oral ring-canal, which is still not blocked up by 

 the caecal tubuli and connective tissue that subsequently occupy it. 



January 27, 1876. 

 Dr. J. DALTON HOOKER, C.B., President, in the Chair. 



The Presents received were laid on the table, and thanks ordered for 

 them. 



The following Papers were read : — 



I. " On the Variations ol the Daily Mean Horizontal Force of the 

 Earth's Magnetism produced by the Sun's Rotation and the 

 Moon's Synodical and Tropical Revolutions." By J. A. 

 Broun, F.R.S. Received December 15, 1875. 



(Abstract.) 



The variations of daily mean horizontal force in the years 1844 and 

 1845 showed several well-marked oscillations, having periods of from 

 20 to 30 days, and amplitudes, in some cases, of more than one thou- 

 sandth of the whole magnetic force. These oscillations were first attri- 

 buted to lunar action ; afterwards they were found more probably due to 

 the sun's rotation on his axis. The disappearance of these oscillations in 



