158 REPORT— 1894. 



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 suf- 

 ficiently 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 fi'ame. 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 mirror- 

 window. 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, liowever, much difi&culty 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 correspond- 

 ing to a known tilt of the instrument. In both cases provision has been 



