AND DN THE T.AAV OF DUESSUUE OF OASES, ETC. 
21 1 
siiitalDly reflected by mirrors, was from ])ebind, and it and tlie lenses were so 
arranged that l)oth points coidd 1)e seen witliout a motion of the liead. Precautions 
were required to prevent tiie radiation from tlie gas flame and h'om the observer 
from producing disturbance, especiallv by unetjual lieating of the two limbs of the Pb 
The U itself was well bandaged up, and between it and the observer were inter¬ 
posed sheets of coj^per mid of insulating material so as to ensure that at all events 
there should lie no want of symmetry in any heating tliat might take place. 
The adjustment itself is a double one, requiring both the use of the levelling 
screw J and an accurate feed of mercury. The hose terminates as usual in a small 
mercury reservoir D. This facilitates the preliminary arrangements, but in use tlie 
reservoir is cut ofl’ by a screw clamp E just below it. The rough adjustment of the 
supply of mercury is eftected by a large wooden compressor F. The fine adjust¬ 
ment required for the actual setting is a more delicate matter. The first attempts 
were by fine screw compressors acting upon the pendent part of the hose, but the 
tremors thence arisino; were found very disturbinq-. A remedv was eventually 
applied by operating u}>on the jiart of the hose which lies flat upon the floor or 
rather on the bottom of a mercury tray. 1'he compressor is shown at G, fig. fl ; tlie 
screw' being provided with a long handle 11 to bring it wdtliin convenient reach. 
The advantage accruing from this small device waiuld scarcely lie credited. 
The glass-w'ork is attached by cement to a board, wfliich hangs dowmw'ards in face 
of the observer and is Itself fixed rigidly to the levelling stand K. This is supported 
at tw'o points I, wdiich define the axis of rotation, and liy a finely adjustable screw' J, 
wdthin reach of the observer. The wdiole stands in a very steady position upon 
the floor of an underground cellar in my country house. 
The arranqemeiits for the connection of the mirror must now be descrilied. The 
glass stems, whose lownr extremities form the “ points,” are prolonged iqiwairds by 
substantial tubing, and terminate aliove in three slightly rounded ends, L, L, suitable 
for the support of the mirror platform N. The tw'o supports iiecessary on the lelt 
are olkained by a symmetrical l)rancl)ing of the tube on that side. The platform is 
of worked glass, so that a slight displacement of the contacts has no effect oil the 
slope of the mirror. Tlie latter is of w'orked glass silvered in front. 8nital)le stops 
are provided to guide the mirror platform into the right position and to prevent 
accidents, but these exercise no constraint. 
The axis 11 about wfliich the ajiparatus rotates is horizontal and parallel to the 
face of the mirror, so that the sine of the anqfle 6 of rotation from the zero nosition 
represents the difference of levels of the mercury surfaces. The axis T I lies approxi¬ 
mately in the mirror suiflace and at about tbe middle of the height of tlie operative 
part. Tlie I'otatioii of the mirror is observed in tbe usual wny liy means of a telescope 
and vertical millimetre scale. The aperture of the object-glass is 30 inllllms., and 
the distance from the mirror 3150 mllllms. The readinqs can be taken tn about 
*1 mlllini. 
2 E 2 
