296 REPORT— 1893. 



on the east side a fine wire. The image of this wire formed by reflexion, 

 first, in the pendulum-mirror, and then in a plane mirror, at c,' is ob- 

 Berved with a telescope, D, furnished with cross-wires and placed in the 

 passage just outside the cellar. A wooden bar, e, is attached at one end 

 to the side of the carriage, and at the other end is graduated to tenths 

 of an inch, the index-pointer being near the floor to the right of the 

 telescope. The carriage is also provided with a screw. motion, which 

 can be worked from the telescope-seat, but it was found unnecessary to 

 use this, as the image can be rapidly and accurately adjusted on the 

 vertical cross-wire of the telescope by gently tapping the end of the 

 graduated bar. 



When the instrument is in order, nearly all operations can be carried 

 on in the passage without entering the cellar. Strings attached to the 

 two sides of the lever are guided by hooks and brought round to either 

 side of the telescope, so that the angular value of the scale-divisions can 

 be determined without leaving the telescope. The gas-jet also can be 

 lighted from the same place. The only occasion for entering the cellar 

 is to readjust the pendulum when one end of the scale-divisions, which 

 occupy a length of about 9 inches on the graduated bar, approaches 

 the index-pointer. But this can be rapidly done by a slight turn of the 

 screw used for tilting the frame. In a few minutes the image becomes 

 steady, and if this be done about the time of day when the tilt is 

 changing in direction, the movement in so short a time will be so small 

 that it may be safely neglected. 



The house in which the pendulam is installed lies on the west margin 

 of Birmingham. The rock is Bunter sandstone, but this is covered at 

 the surface by a thick bed of clayey sand, with occasional layers of 

 rounded pebbles. The nearest edge of the road in front of the house is 

 11 yards from the pendulum. A single-line suburban railway runs in a 

 deep cutting at the bottom of the garden, about 80 yards from the 

 pendulum. But no disturbance has ever been noticed from either pass- 

 ing carts or trains, the short vibrations or tremors produced by them 

 being too rapid to affect the pendulum, and the local bendings of the 

 ground caused by their weight apparently having no effect. 



The Foundation of the Pendulum.^ — A circular hole, 3 feet in diameter, 

 was made to a depth of 8^ feet. The bottom of the hole is covered with 

 a layer of concrete, C (fig. 5), 2 ft. 2 in. in thickness, and on the surface 

 of this is placed vertically a cast-iron water-pipe, A, 6 ft. 6 in. long, 

 \0^ inches in external diameter, and ^^ inch thick, and weighing 

 about 3 cwt. ; so that the pipe projects about 2 inches above the cellar 

 floor. A further layer of concrete, nearly 2 feet in thickness, sui-rounds 

 the bottom of the pipe and keeps it in position, and the pipe itself is also 

 filled with concrete to within 2 feet of the top. The pendulum stands 

 on an iron plate, B, which rests at three points only on top of the pipe ; 

 lateral displacement is prevented by pieces projecting a little way down 

 the inside of the iron pipe. In order to isolate the stand from the cellar 

 floor, the iron pipe is surrounded by earth only to about 2 feet from the 

 top, and then an earthenware drain-pipe, D, is placed surrounding the 

 pipe, leaving an air-space between. The interval between the drain- 

 pipe and the sides of the hole is filled with earth, and the brick pave- 



' This mirror is an ordinary plate-glass looking-glass. It should, however, be 

 made of worked glass, as should also the window of the mirror-box (b, fig. 2). 

 ^ This foundation was designed by Mr. Horace Darwin. 



