uly 1, 1886] 
the direction of propulsion, the current urging it will be in the 
direction of twist in the muscles of the wrist. In treating the 
movement of a conductor carrying a current in the magnetic 
field, I have used a rule identical in character with Ampere’s, 
and that was probably the rule to which J. T. B. referred in his 
critique, namely, that a figure swimming in the current and looking 
along the lines of force is carried to his left. I should be glad 
to find a rule at once as complete and more simple, although 
after a pretty wide experience, not always with the very brightest 
of pupils, I have not been sorely pressed with the difficulty 
J. T. B. seems to have felt, All the required attitudes are pretty 
familiar toa boy who is accustomed to diving in the water and 
swimming on his front, side, or back. L. CUMMING 
Rugby, June 24 
THE following version of Ampere’s Rule is one which I com- 
municated some time ago to a few friends, but it did not appear 
to me to be expressed in language sufficiently grave to justify its 
publication. Still, as the Rule is a simple and useful one, your 
readers, in general, may be disposed to overlook its levity. 
Draw the three well-known Hamiltonian vectors, i, j, k. 
_ After i put (nduction), after j put (ump), and after k put (urrent). 
Then the figure explains the action of magnetic induction on an 
electric current. The figure in fact asserts that i(ndaction) in i 
makes k(urrent) in k to j(ump) along j. 
k(urrent) 
os j(ump) 
J 
/ 
| va 
| Ya 
“ 
i(nduction) 
__ Of course the same figure gives the direction (according to the 
‘Law of Lenz) of the current yenerated by a motion (¢.e. a jump) 
of a conductor in a given direction in a magnetic field in which 
the direction of the induction is given. 
R.LE. College, Cooper’s Hill 
| 
| ae 
GEORGE M. MINCHIN 
An Earthquake Invention 
| 
| In my letter to Nature, vol. xxxiii. p. 438, I clearly showed 
‘that the supposition of Mr. D. A. Stevenson and Prof. Piazzi 
myth that I hadendeavoured toclaim the aseismatic joint of Mr. 
D. Stevenson was due to their imperfect acquaintance with seismo- 
logical literature. I certainly intercalated a ote about aseis- 
matic structures in a report to the British Association on earth- 
quake phenomena in general, without mentioning Mr. Stevenson’s 
name. 
Previous to this, when specially speaking or writing upon 
aseismic structures, I have repeatedly referred to the work of Mr. 
D, Stevenson. Such references were quoted. Under the cir- 
cumstances I asked Messrs. Stevenson and Smyth to distinctly 
state whether they still considered themselves justified in con- 
tinuing their accusations. If this point was overlooked the dis- 
cussion might be considered as at an end. Mr. D. A. Steven- 
son has replied, but the question at issue as been distinctly 
evaded (NATURE, vol. xxxili. p. 534). 
I deeply regret that Messrs. Stevenson and Smyth should 
allow a discussion to terminate in such a manner. 
Tokio, May 22 JOHN MILNE 
[This must now close.—Ep. NATuRE.] 
Professor Newcomb’s Determination of the Velocity of 
Light 
| I HASTEN to correct an error which has crept into my 
/account in last week’s NATURE (p. 171) of Prof. Newcomb’s 
measures of the velocity of light. ‘The arrangement employed by 
Foucault in 1862 was vot that adopted by Newcomb, and illus- 
trated in Fig. 1, but that sketched in Fig. 2. In other words, he 
Dlaced his lens between the revolving and fixed mirrors. His 
apparatus is described in Comptes rendus, t. lv. P- 792, where 
he velocity of the rotating mirror is stated to have been 400 
NATURE 
193 
revolutions a second, and the total length of path between the 
mirrors 20 metres. A. M. CLERKE 
June 28 
Solar Halo and Sun Pillar seen on June 5, 1886 
WHEN approaching the Observatory, about 6.45 p.m., my 
attention was drawn to portions of a solar halo, which appeared 
as in Fig. 1. 
HORIZON 
Fic. 1.—a, very bright ; c, fainter; B, very faint. 
This remained visible until after 7 p.m., and nothing more 
was seen before 7.30 p.m. When looking out at 7.40 p.m. 
G.M.T., I noticed something unusual, and came at once to the 
conclusion that it wasa solar pillar, and made a sketch in a 
note-book and the following remarks :— 
HORIZON 
Fic. 2. 
The shaded part in the foregoing, other than the halo, pillar, 
and stratus cloud, represents cirrus. 
The pillar apparently rose from the sun, which—when I 
looked out at 7.40—kad just gone below the top of some dark 
stratus cloud, directly to the upper part of the halo marked a. 
It was not more than 10° high at the brightest, but quite as 
much, as I estimated it to reach zearly half-way to the portion of 
the halo A, and the width four times the diameter of the sun. 
The lower part of the pillar was well defined and of a golden 
colour ; as it approached the halo it gradually became fainter, 
and was then lost in the cirrus cloud. ‘The upper part was some- 
what wider ; perhaps this was duc to the greater amount of cloud 
there, which diffused the light. 
At 7°55 G.M.T. all portions of the halo had gone except a 
small piece at A, and the pillar was fainter, but still quite 
visible. At 8°3 the halo and pillar had disappeared. ‘lh- sun 
set at 8°14 p.m. 
An ordinary hal» (224°) was visible, more or less bright and 
complete nearly from sunrise to sunset. 
1 could not fix the position of the pillar by stars, none near 
being visible. The sketch was made at the time from a window 
of the library of the Observatory. 
That seen here on 1883 April 6, by Mr. W. A. Robinson, 
was about I5 minutes affer sunset; this observation was 30 
minutes efore. The time at which the pillar was v.sible on the 
former date was given by nearly all your correspondents as 
