28 
We thus obtain 
: - F2 : at Y : 
Sietths Tae o=5(1 3 = 
2b = 0°2308 = 2°77 inches. 
J’ = 0°2998 = 3°60 ,, 
M = 240. 
@ = 13°46. 
whence 
‘«‘ The field here is nearly the same as in the Cassegrain ; 
with the single lens it is something larger, which arises from 
that lens acting differently in the two cases, in that it dimi- 
nishes the image, and therefore requires an increased value 
of d’.” 
DEcEMBER 12TH, 1853. 
THOMAS ROMNEY ROBINSON, D.D., Presipent, 
in the Chair. 
Tue Rey. H. Luoyp, D. D., read a supplemental note “ on 
the magnetic influence of the moon.” 
Ina previous communication* the author had shown, from 
a discussion of the observations made at the Magnetical Ob- 
servatory of Dublin, that the magnetic declination was subject 
to a small periodical variation dependent upon the moon’s 
hour-angle, the north pole of the magnet deviating twice to 
the east, and twice to the west, in the course of the lunar day. 
It was, of course, to be expected that a similar variation would 
be found to affect the other two magnetic elements. In order 
to trace its existence, and to determine its law, in the case of 
the horizontal component of the magnetic intensity, the author 
* Proceedings, May 9, 1853. 
