278 
NATURE 
OUR ASTRONOMICAL COLUMN 
SOLAR PARALLAX FROM OBSERVATIONS OF MARS,—In an 
appendix to the Washington Observations for 1877, Prof. East- 
man, of the U.S. Naval Observatory, deduces ‘*a value of the 
solar parallax from meridian observations of Mars at the oppo- 
sition in 1877.” In September, 1876, a circular was addressed 
from Washington to the principal observatories in both hemi- 
spheres, inviting co-operation in systematic meridian observa- 
tions of Mars at the close opposition of the following year, and 
in response series were received from the Cape of Good Hope, 
Melbourne, Sydney, Cambridge, U.S., Leyden, Kremsmunster, 
and San Fernando, but Prof. Eastman excludes from his inves- 
tigations the observations at the last two observatories, in the 
absence of sufficient details as to the methods and instruments 
employed. In the circular it was proposed to follow virtually 
the method of observation adopted at Pulkowa, by Prof. Win- 
necke in 1862, but it is stated, ‘‘ The prescribed method of 
observing was fwély carried out at only /wo stations and fartially 
atone. Where the plan of the circular was strictly followed, 
the character of the work was decidedly superior to that where 
the directions were disregarded.” 
The results of the comparisons are thus given :— 
Sun’s No. of 
Parallax. | Comparisons. 
Washington and Melbourne : 8-071 me) TO 
Washington and Sydney ... SB8hie ces 7 
Washington and Cape of Good ‘Hope 8896... 7 
Melbourne and Leyden ... ... ... 8'°969.. 27 
Melbourne and Cambridge, U.S. ... 91138 ... 10 
With respect to the large value of parallax given by compari- 
son of Melbourne and Cambridge, Prof. Eastman remarks : 
‘© This difference may arise from the method of observing over 
inclined threads at Cambridge, for the agreement of the results 
among themselves is very satisfactory ; but, whatever the cause 
of the discrepancy may be, it has not been deemed advisable to 
employ these values in obtaining the final result.” 
‘The mean of the remaining sixty results, with regard to the 
computed weights, gives for the solar parallax, 8""953 + 0”*o19. 
It has been assumed that this method of determining the sun’s 
parallax is certain to give too large a value, and Mr. David Gill, 
now H.M. Astronomer at the Cape, has suggested a definite 
cause; but Prof. Eastman, after experimenting upon Jupiter, 
does not find in his case that Mr, Gill’s theory holds good. He 
intends, however, to pursue the investigation upon the disk of 
Mars. 
VARIABLE STARS.—An ephemeris of the variable stars, similar 
to those of previous years, has been issued by the ‘‘ Astronomische 
Gesellschaft” for 1882. It contains the times of maxima and 
minima of most of the variables whose periods are known, in- 
cluding, in addition to Algol, five stars of the Algol-type, viz. 
A Tauri, S Cancri, § Libra, U Coron, and U Cephei. A 
minimum of Mira Ceti is fixed to February 3—this phase has 
been much less observed than the maximum. Both this minimum 
and the following maximum on May 23 are dated about ten days 
earlier than Argelander’s formula of sines would indicate, but 
the observations of the last ten years have shown additional per- 
turbation. A minimum of x Cygni is dated February 20, and a 
maximum on August 25. The following are Greenwich times of 
minima of Algol :— 
h, m. hm | h. m. 
Feb. 1, 8 28 March 10, 15 4 April 2, 13 35 
15, 16 33 13, 11 53 | 5, 10 24 
18, 13 22 16, 8 42 Boy 5) 07 
21, 10 10 | ZEt2) 6 
24, 659 28, 8 55 
Minima of S Cancri occur February 16 at 11h. 23m., March 7 
at 10h. 38m., March 26 at gh. 54m., and April 14 at gh. 9m. 
For U Cephei (Ceraski’s variable) calculated times of minima 
are :— 
h. m. hm. | h. m. 
Feb. 1, 15 8 March 3, 13 3 April 2, 10 58 
6, 14 47 | 8, 12 42 7, 10 38 
II, 14 26 13) 12122 12, 10 17 
16, 14 6 16; 02x 17, 9 56 
21, 13 45 23, II 40 | 22,935 
26, 13 24 28. ir, 19 a 27a) 15 
A minimum of U Corone is dated February 6 at 10h. 7m. ; the 
period is 3d. 10h. 51°24m. ; the extent of variation about one | 
magnitude, 
THE ROYAL ASTRONOMICAL SOCIETY.—We are happy to be 
able, on the authority of Prof. Winnecke, to correct a mis-state- 
ment in this column, referring to the deceace cf M. Gautier as 
leaving Prof, Plantamour the senior Associate on the list of this 
Society, Notwithstanding some reports to the contrary, Prof. 
Winnecke informs us that this position is occupied by Prof. 
Rosenberger, who is still alive and in good health. Forty-five 
years have elapsed since the Society’s gold medal was presented 
to Prof. Rosenberger, at the hands of Sir George Airy, for 
his masterly and elaborate researches on the motion of Halley’s 
Comet. He was elected an Associate in April, 1835. 
THE GREAT Comet oF 1881.—On January 7 and 8 Prof. 
Winnecke obtained good determinations of the position of this 
comet, which is still well observable with the great refractor at 
Strasburg. Its apparent diameter was about 30”, and there was 
a condensation presenting the brightness of a star of 13m. The 
resulting places are— 
M.T. at Strasburg. R.A. Decl. 
h. m. s. h. “ms. all wie 
January 7 749 6 22 50 21°70 +57 48 59°0 
8 7 333 22 52 49°72 +57 42 1572 
It will be seen that Dr. Dunér’s ephemeris in the Astronomische 
Nachrichten still gives the comet’s position pretty closely. 
THE DETERMINATION OF ELECTROMOTIVE 
FORCE IN ABSOLUTE ELECTROSTATIC 
MEASURE 
AVING already described my absolute sine electrometer 
before the Physical Society and at this year’s meeting of 
the British Association, there is no necessity for describing here 
more than the prominent features of the instrument. ‘Two 
plates of brass, each about one foot square, their surfaces being 
rendered true planes, are connected together, as a rigid body, by 
four ivory axes passing through both plates near their corners. 
On these axes are placed (between the plates) washers of mica, 
which serve to keep the plates asunder and parallel at a very 
small distance from each other. One of the plates is con- 
tinuous ; the other (the guard plate) has in its centre a square 
aperture whose side is 3 centimetres Jong, and in this aperture 
hangs a very light disk of aluminium suspended from the top of 
the guard plate by two Wollaston platinum wires each about 7°5 
inches long. The disk is flush with the guard plate when it rests 
against four fine screws attached to the latter. The system of 
plates is movable, as a rigid body, round a horizontal axis, and 
its motion is produced by a micrometer screw (1-16th of an inch 
pitch) working against an insulated portion of the lower edge of 
the continuous plate ; thus the screw tilts the system out of the 
vertical to a measurable amount. The horizontal axis of the 
plates is carefully levelled with a cathetometer, and the exact 
distance between the plates is determined by three readings of a 
spherometer taken at the aperture of the guard plate (previous to 
the insertion of the disk) before the mica washers are inserted 
between the plates (the plates being in complete contact), and 
three readings at the same points after the insertion of the 
washers. The vertical distance between the centre of the axis 
of plates and the point of the micrometer screw is 15 inches; 
the weight of the disk ‘2568 grammes; and the head of the 
micrometer screw is a circle 3 inches in diameter, divided into 
1000 equal parts. 
The essential principle of the instrument will be understood 
from the following figure. B is the horizontal axis about which 
the plates C (the continuous plate) and G (the guard plate) are 
tilted by the fixed micrometerscrew A. The disk is represented 
by the full line D in the centre of the guard plate. 
To measure the E.M.F. of a battery, put C in connection 
with the positive pole, while the negative pole and the guard 
plate (and, with it, the disk) are connected with earth, 
If N is the attraction exerted on the disk by the charge on C, 
W=weight of disk, @=angle of deflection of the plates from 
the zero, or vertical, position, we shall have, when the disk is jus# 
out of contact with the little screws which keep it flush with the 
guard plate, 
N= W sini eon Rese pe (0) 
For the particular instrument which has been constructed for 
(Fan, 19, 1882 — 
