170 CAPTAIN A. E. CLAEKE ON THE COMPAEISON OF ENGLISH AND FOEEIGN 
designated EE'; (2) at 2 inches right and left of the centre designated CC'; (3) at 
inches right and left of the centre designated NN'; (4) at ^ 6 - inches right and left 
of the centre designated SS'. 
The following Table contains the observed and computed changes of length of one of 
the bars, of its whole length, and of its subdivisions ; the dots are marked in order from 
left to right, m, n, n', ml ; A(mn') is the alteration of the distance mn' due to the change 
of supports ; the unit is the millionth of a yard. 
Changes of supports. 
A [mn'\ 
A \mm'\ 
A [nm'\. 
From 
To 
Observed. 
Computed'. 
Observed. 
Computed. 
Observed. 
Computed. 
NN' 
EE' 
57*8 
58-6 
70-7 
71-8 
60-7 
58'6 
EN' 
25-0 
26-3 
25-9 
26-4 
17-4 
16-8 
>5 
NE' 
16-4 
16-8 
26-8 
26-4 
28-5 
26-3 
CC' 
-36-2 
—35-3 
-35-8 
-35-7 
-35-7 
-35-3 
y> 
NC' 
-31*8 
— 30-0 
-31-8 
— 30-4 
-322 
-30-1 
„ 
CN' 
-31*7 
—30-1 
-31-8 
-30-4 
-32-4 
-30-4 
CC' 
EE' 
93-5 
93-9 
106-5 
107-5 
93‘4 
93-9 
The modulus of elasticity by which the computed results are obtained is derived from 
the observations themselves. 
Absolute Expansion . — The coefficients of expansion of the Indian standards I s , I B and 
of the two other 10-foot bars of iron, QI n OI 2 , have been obtained by means of an appa- 
ratus constructed for the purpose. Theoretically it is a simple matter to determine the 
coefficients of expansion of two bars A, B ; it may be done as follows ; — Compare A hot, 
say at a steady temperature of 100°, with B at the temperature of, say 40° ; next compare 
A at 40° with B at 100° ; and lastly compare the bars when both at the same tempera- 
ture. But the practical difficulty is to maintain a steady temperature for the hot bar, 
so that it shall not be cooling while under observation. This has been effected in the 
following manner: — Imagine two closed tanks of copper measuring 124 inches by 5 by 
3 inches, and suppose them fixed to the upper surface of a stout mahogany plank of the 
same length and 8 inches broad ; between the tanks there remains a vacant space 2 inches 
wide and 5 inches deep ; into this space the bar goes with its supporting-rollers, which 
are capable of slight vertical movement for level or focus adjustment. A current of hot 
water at a steady temperature enters the bar-room from without by a flexible tube ; this 
current is made to subdivide into four equal streams, entering each tank by two orifices 
in its upper surface, at one-fourth and three-fourths of the length. The water escapes 
from each tank by an orifice at the bottom of either extremity, and is conducted out of 
the bar-room through flexible tubes. The supply being purposely greater than can be 
carried away from the extremities, an overflow pipe is provided at the centre of the 
tanks, and this overflow is also carried away from the room by flexible tubes. Thus a 
constant circulation of water is maintained, and no part of the water in the tanks can be 
still or cooling, nor can the tanks empty or overflow. The whole is well wrapped up in 
