62 Sir William Siemens on the 
These lines intersect the triangles (fig. 1) at their angles, and 
also at the bisection of their sides. At these points the mo- 
tion has been already determined. The motion is circular if 
p is an even multiple of ^, at the points for which 1 — A= + B 
— that is, where q— r = 2in + ^ {m being an integer); and if 
p is an odd multiple of ^, at the points for which l+A= +B 
— that is, where j-r=2m + |. 
These conditions are satisfied at the middle points of the 
triangles. In fig. 2 are shown the nodes and the circular 
points, the arrows indicating the phase when t = 0. It will be 
noticed that at adjacent circular points the motion is in oppo- 
site directions. 
It would be possible to construct a piece of apparatus to 
exhibit the motion approximately. A piece of elastic mem- 
brane, sufficiently stretched in all directions, should be fastened 
at a set of points corresponding to the points of rest, and the 
middle points of the triangles should then be displaced accord- 
ing to the phase (see fig. 2), and carried round their original 
positions in circles of ecpial size and period, the adjacent mo- 
tions being in opposite directions — an arrangement which 
might easily be effected by a series of cogged wheels. We 
should then have a number of points fixed, and the correct 
motion given at other points where the motion is greatest. 
The motion of the rest of the membrane except near the edges 
would then be approximately correct. 
In fig. 3 is given an enlarged view of one of the triangles, 
showing some of the points where the motion is elliptic, and 
the displacement of the lines through the nodes parallel and 
perpendicular to the rays. 
IX. On the Conservation of Solar Energy. 
Reply by Sir William Siemens to Mr. E. H. Cook*. 
ARTICLE LX. in the June Number of the ' Philosophical 
Magazine/ by E. H. Cook, B.Sc, calls for a reply to 
some of the objections raised against my Solar hypothesis, 
which I am the more readily disposed to give, inasmuch as 
they differ from those already raised by others, and involve 
moreover questions of general interest. Mr. Cook proves 
that C0 2 is distributed uniformly throughout our atmosphere; 
and concludes that the power of gaseous diffusion is such that, 
admitting (as he does) a universal plenum, the same gaseous 
proportion must prevail throughout space — that, in short, 
there must be as large a proportion of C0 2 and X in space 
* Communicated by the Author. 
