142 On Mr. Ferrel's Theory of 
Distance between the centres of the plates 90 centimetres. 
Position of Galvanometer- 
plates, deflection. 
| | 633 
(II ™° 1 
V. 1 \ 634 V 
I | — 637 J 
r | — 660 -v 
r I. \ I \ 655 
I | | 651 3 
between the centres of the plates : 
C\ ~ 403 } 
VJL 1 | 368 
I I 358 
Temperature. 
15° C. 
Distance between the centres of the plates 20 centimetres. 
403 
15° C. 
Battery reversed. 
| 208 
II. { \ 211 
| | 229 
Here again, then, the resistance is least with the plate end 
on, even when the distance between the centres of the plates 
is as much as 80 centimetres. 
This apparent anomaly of the smaller resistance obtained 
when one or both plates is put end on is, as w r as pointed out 
by Mr. Boys, probably due to the smaller density of the gas 
which is deposited on a plate when it is put end on (in conse- 
quence of the current flowing from both sides of the plate into 
the liquid under these circumstances) more than compensating 
for the want of parallelism of the lines of flow when one or 
both of the plates are put end on. 
XXI. On Mr. Ferrel's Theory of Atmosp/ieric Currents. 
By Professor J. D. Everett, F.R.S.* 
MR. D. D. HEATH, in his attack upon Mr. Ferrel in 
the July number of the Philosophical Magazine, over- 
looks the well-known principle of Conservation of Areas 
for the case of a particle acted on by a force which always 
passes through a fixed line. In such a case, the projection of 
the particle on a plane perpendicular to the fixed line describes 
equal areas in equal times about the fixed line. 
If the particle is constrained to remain on the surface of a 
smooth sphere, and is acted on by a force always directed to 
the centre of the sphere, the property holds with respect to 
* Communicated bv the Author. 
