We see, therefore, that the amplitude of the deviations is greatest during the summer months. It will be remarked that the first and last series, corresponding nearly to the same times of the year, give almost identical deviations.

APPENDIX II.

The Bifilar Pendulum at the Royal Observatory, Edinburgh. By Professor R. COPELAND, F.R.S.E., Astronomer Royal for Scotland.

This instrument was placed in position by Mr. H. Darwin on March 23, 1894. It is, with slight exceptions, similar to the instrument erected at Birmingham, in April 1893, for Mr. C. Davison, and which has been fully described in the British Association Report for 1893, pp. 291-303. The exceptions referred to are: (1) the arrangement of the mirror of the Edinburgh instrument at right angles to the plane of the suspending wires, and (2) the surrounding of the instrument by a heavy casing to prevent, as far as possible, any movement of the mirror due to change of temperature resulting from the lighting of the illuminating lamp or other cause.

The whole apparatus, including the scale and lamp, is placed in a hut erected for the purpose over a trench running east and west, formed within the Observatory grounds by removing the soil and levelling the rock. A hole 2 feet deep is bored in the rock near the west end of the trench, and into this is leaded the heavy iron supporting bar. This bar is 1 inch in diameter, and projects above the surface of the rock sufficiently far to allow the iron plate carrying the instrument to be fixed to it by screws. A slate slab 18 inches square and 1 inch thick, in which a circular hole is cut 9 inches in diameter, is placed round the iron plate, but not touching it, and is supported on a wooden frame at the level of the plate. On this slate is placed a square cast-iron casing, 6 inches deep, which surrounds the body of the instrument, including the mirror-box. On this, again, rests a second slate with a circular hole, 6 inches in diameter, through which passes the head of the instrument and the upper part of the brass tube containing the frame. The head is covered by a stoneware jar resting on the upper slate.

The cast-iron casing surrounding the body of the instrument is perforated by an aperture 4 inches in diameter opposite the mirrorwindow. To prevent undue access of air, and consequent dewing of the window, a truncated cone of sheet copper is fastened inside this aperture, with its smaller end turned inwards towards the instrument. In spite of this precaution, however, much difficulty has been experienced from the dewing of the glass, and a wooden shutter lined with green baize has been arranged to still further prevent the circulation of air. The shutter is raised when necessary by pulling a string from the east end of the hut. In addition vessels containing chloride of calcium have been placed in the hut and inside the cast-iron casing. Much benefit has been derived from these arrangements. The casing is also perforated with holes to admit the handles of the tangent screws and the pipes of the bellows used for ascertaining the number of divisions of the scale corresponding to a known tilt of the instrument. In both cases provision has been

made for preventing tremor being conveyed to the instrument by the use of these parts of the apparatus, the whole of such tremor being taken up by the casing.

The scale and small benzoline illuminating lamp are placed at the east end of the hut. A frame, supported in a horizontal position by two strong iron feet fixed by beds of cement laid on the rock, is traversed by the lamp-stand, which carries an index along the edge of the scale. The readings are taken when the image of a wire placed vertically in front of a circular hole in the lamp-screen coincides with the vertical wire in a fixed theodolite. The scale is divided into millimetres, and the effective length of it traversed by the index is 382 mm., and it is distant 10 feet (=3,048 mm.) from the centre of the instrument. If be the distance of the lamp from the point of the scale which is due east of the mirror, the azimuth, north or south of east, of the normal to the mirror is tanWhen l=191 this becomes 1° 47′ 35", and represents the rotation of the mirror to each side of the north and south position within the range of the scale. When the rotation exceeds this amount the mirror has to be brought back to its north and south position by turning the long handle attached to the southern levelling screw.

3048

The plane of the suspending wire is in the east and west direction with the longer section of the wire toward the east; consequently, as the face of the mirror is towards the east, a tilt of the upper support of the wire towards the north produces a corresponding deflection of the normal to the mirror also to the north; hence tilts in the north and south direction only are measured by the instrument.

=

From measures of the dimensions of the instrument which have been supplied by Mr. Horace Darwin, it is computed that the movement of the lever, attached to the micrometer screw against which the top of the frame is pressed, through the fixed amount of 13 mm. for which the apparatus is set, produces a tilt of 2016 seconds of arc in the upper support of the suspending wire. As soon as the pendulum was mounted experiments were made to ascertain its sensitiveness, or the scale value of this fixed amount of tilt. The mean of nine measures taken on March 26 gave 62.4 mm. of the scale 2016. This was considered excessive, and steps were taken to reduce it gradually. The mean of four measures made between May 5 and 8 gave 21.2 mm. = 2016. Since the last of these dates the two levelling screws, whose combined movement alters the sensitiveness, have not been interfered with. Observations of the sensitiveness have, however, been made occasionally, four measures between May 16 and June 9 giving 19.3 mm., 17.2 mm., 200 mm., and 23.7 mm. respectively, or an average of 200 mm., equal to 2"-016. Since June 9 four measures have been made, but these give somewhat anomalous and as yet unexplained results.

In the absence of any photographic arrangement for giving a continuous record of the position of the mirror, it has been decided to take readings at each full minute from five minutes before to five minutes after Paris mean noon every day. This has been carried out from May 26 up to the present time, with the exception of a few days, when the readings were rendered impossible by a deposit of moisture on the mirror window or other cause. These observations were made by Mr. T. Heath and Mr. A. J. Ramsay.

On no occasion has any unsteadiness or oscillation of the mirror been

observed, though a slight change of position appears to take place during the time of observation, owing possibly to the presence of the observer or the heat of the lamp. This change, however, is very slight, and the mean of the eleven observations of each set is taken as showing the position of the mirror for that day at Paris noon. These mean scale readings have been laid on a curve with the date as one argument and the divisions of the scale as the other. The result shows that the mirror has been constantly turning in azimuth from the east towards the north during the whole period over which the observations have extended (May 26 to July 31). The total amount of this movement has been 517 mm. on the scale in sixty-six days, or an angular rotation of 9° 37′ 23′′ for the ray falling on the mirror from the lamp; which is, of course, equivalent to an angular rotation of 4° 48′ 42′′ in the mirror itself.

If, now, 20 mm. (=2"-016) be taken as the sensitiveness of the instrument over all this time-though about this number there is some uncertainty-the total tilt of the instrument towards the north appears to have been fifty-two seconds of arc in sixty-six days. The stability of the nadirpoint of the mural circle in the adjoining observatory proves that this tilt must either be in the superficial layer of the rock to which the instrument is attached or, which is far more probable, in the pendulum itself.

The experiments confirm the results obtained elsewhere, that the instrument, while unfitted to show the slower progressive tilts of the earth's surface, is pre-eminently suited, by its great sensitiveness and momentary stability, for the indication of earth tremors. However, to bring out the full powers of the apparatus it is obviously necessary to secure a continuous photographic record of the position of the mirror.

M. Antoine d'Abbadie, at whose cost the pendulum was supplied, has eaused simultaneous observations to be taken with his 'nadirane' at Abbadia in north latitude 43° 22′-8 and longitude 7m. Os. west of Greenwich. The readings at the two stations have, however, not yet been compared.

The Electrolytic Methods of Quantitative Analysis.-Report of the Committee, consisting of Professor J. EMERSON REYNOLDS (Chairman), Dr. C. A. KOHN (Secretary), Professor P. FRANKLAND, Professor F. CLOWES, Dr. HUGH MARSHALL, Mr. A. E. FLETCHER, Mr. D. H. NAGEL, Mr. T. TURNER, and Mr. J. B. COLEMAN.

THE first work undertaken by the Committee has been the compilation of the bibliography of the subject, with which some progress has been made.

In addition, the plan on which the experimental part of the work is to be carried out has been arranged. This is to include the investigation of the methods for the determination of the following metals: silver, lead, mercury, bismuth, cadmium, tin, antimony, iron, zinc, manganese; and subsequently of the methods for the separation of these metals both from one another and from other metals.

This is all the Committee undertook to do when they were appointed without a grant of money.

They now ask to be reappointed and with a grant of 401.

Bibliography of Spectroscopy.-Report of the Committee, consisting of Professor H. MCLEOD, Professor W. C. ROBERTS AUSTEN, Mr. H. G. MADAN, and Mr. D. H. NAGEL.

THE work of searching for, and arranging chronologically under proper heads, the titles of papers on subjects relating to spectroscopy has been proceeded with by the Committee, and a list is appended which brings the catalogue of spectroscopic literature up to the beginning of 1894.

It will be a subject for consideration whether the reappointment of the Committee would be advisable. Considerable difficulty arises in finding any one who can gratuitously devote sufficient time to the work of obtaining and verifying references to papers, and who possesses at the same time the requisite facilities for doing so.

In the meantime, however, the Committee ask to be reappointed for one more year.

PAPERS ON SUBJECTS CONNECTED WITH SPECTROSCOPY.

Continuation of the List published in the Report for 1889.

[In cases where it has not been found possible to verify a reference, the latter is placed in brackets, in the same column as the title of the paper. A list of the chief abbreviations used will be found at the end of the catalogue.]

W. E. Wilson

1883.

Verbessertes Prisma à vision
directe.' (Read April 23.)

'Ber. Erzb. Haynald'schen Obs. zu Kalocsa in Ungarn,' 1883, 133-138, 'Zeitschr. f.' Instrumentenkunde,' vii. 399-400 (Abs.); Beiblätter,' xii. 335-336 (Abs.)

A Reflecting Spectroscope. (Roy. Nature,' xxix. 167 (Abs.)
Soc. Dublin, Nov. 19.)

R. von Kövesligethy Ueber ein neues Kolorimeter, zu

gleich Spectralphotometer. ('Cen-
tralzeitung f. Opt. u. Mech.' vi.
55.)

1894.

M

C. C. Hutchins

N. von Konkoly

A New Photographic Spectroscope. Amer. J. Sci.' xxxiv. 58(July).

Ein Siderospectrograph. (Feb.). 'Centralzeitung f. Opt. u.

Th. W. Engelmann. Das Microspectrometer. (June). Zeitschr. f. wiss. Micro

Universal-Spectralapparat. (June) Zeitschr. f. phys. u. chem.