15(5 
DU, A. E. H. TUTTOX OX THE ELASMO^IETEE, 
A support for the pan is provided to prevent it swinging and to remove strain 
from the beam when not in use. It consists of a short, hollow cylindrical column, 
with heavy solid base covered underneath with cloth, carrying at its summit a 
rotatable but verticallj^ immovable nut with milled flange ; geaiing within is a screw 
carrying at its head a table of ebonite, hollowed to the shape of the under part of the 
pan. When the acce.ssory is placed under the pan, the table can be raised to support 
the latter by rotation of the milled nut. 
A removable support is likewise provided for the counterpoise on the left, in the 
shape of a little tal)le of the diameter of the cylinder, which can be raised to support 
the latter by means of a similar milled nut carried, however, at the end of a horizontal 
arm whose other end is a doubly split, and therefore detachable, collar grijiplng 
round the column of the control apparatus described m the next section. 
The determination of the position of the interference bands at the conclu.sion of 
an observation, when the control is' removed and the full weight on the pan is 
operating in bending the jDlate, is considerably disturbed by flickering of the bands, 
unless absolute quietude is obtained and absence of all vibration. The difficulty can, 
Ijowever, be entirely overcome by fixing a horizontal disc below the pan and immersing 
it in oil. A disc of aluminium is provided, of the same diameter as the pan, to which 
it can be rigidly fixed by means of a stout aluminium rod, 3’5 centims. long; one end of 
the rod is screwed firmly into the centre of the disc, washers above and below aiding 
file rigidity, and the other end screws into a little boss fixed permanently underneath 
the bar on which the pan is mounted. It is thus readily detachable if not required. 
Tlie disc, when in use, is immersed in oil, cedar oil being found admirable for the 
})urpose, contained in a glass dish with perfectly parallel sides and of a slightly 
larger diameter than the disc, which, of course, mu.st not touch the glass. 
The resistance which the oil offers to tlie vertical motion of the disc effectually 
steadies the bands, so tliat the final position can be measured with the utmost 
accuracy. 
Cedar oil possesses the two advantages of not soliditying, and of being so slightly 
viscous as scarcely to impair at all the sensibility of the balance. 
Tlie amount of vertical motion of the beam ends and the pan is so slight, not 
amounting to a millimetre from first to last if the su])porting frame is kept in action 
up to the moment of beginning the bending, that the amount of the rod which is 
wetted liy the oil is practically constant. It is therefore found sufficient to add to 
the counterpoise on tlie left side a weight, previously determined with the greatest 
care, and made in the form of a platinised disc, which precisely balances the 
aluminium fitting as immei’sed in oil. It is laid on the top of the cylindrical compen¬ 
sator, and has a radial notch to enable it to fit round tlie suspending rod. Any slight 
remaining ine(pialitv of the weights on tlie two sides of the balance is not more than 
can be corrected by slight moyement of tlie adjustment vane carried above the centre 
oftliefieam 
