91 
Mr. Graves, Fellow of Trinity College, mentioned the fol- 
lowing theorem of his own, relative to confocal surfaces :— 
“If there be two confocal surfaces of the second order, 
one of which is circumseribed by. a cone whose plane of 
contact touches the other surface in a certain point, the 
normal at this point will pass through the vertex of the cone.” 
A new rain-gauge was exhibited to the Academy, con- 
trived by the Rev. Thomas Knox. 
The object of this instrument is to register the amount 
of rain that falls when the wind is in different points. Its 
construction is very simple. The water,—instead of descend- 
ing from the reservoir directly into the tube of registry,— 
passes through a lateral tube into an annular-shaped vessel, 
divided into eight compartments, each .of which terminates 
below in a graduated glass tube. It is obvious, then, that 
if the eight tubes be set to correspond with the cardinal and 
intermediate points, and that the reservoir be made to re- 
volve on a vertical axis by means of a vane, the direction of 
which corresponds with that of the lateral tube; the object 
proposed will be attained. Mr. Knox has preferred to make 
the reservoir fixed, and the system of tubes moveable; but 
the result is obviously the same. 
The Academy then proceeded to consider the amend- 
ments of the by-laws, proposed by the Council; and, after 
some discussion, it was resolved that there be an adjourned 
meeting at two o'clock, on Friday the 30th of June, for the 
further consideration of the subject. 
DONATIONS. 
A Copy of the Ordnance Survey of the County of Lei- 
trim, in 40 sheets. Presented.by Lieutenant-Colonel Colby. 
Proceedings and Ordinances of the Privy Council of Eng- 
land, Vols. VI. and VII. Presented by the Commissioners 
of the Public Records. vA. 
