of Edinburgh, Session 1885-86. 
685 
fixed end. In this position the equilibrium is very nearly unstable. 
A definite sighted position has been chosen for the weight, 
relatively to a mark rigidiy connected to the fixed end of the spring, 
fulfilling the condition that in this position the equilibrium is stable 
at all the temperatures for which it has hitherto been tested ; 
while the position of unstable equilibrium is only a few milli- 
metres above it for the highest temperature for which the instru- 
ment has been tested, which is about 16° C. 
The fixed end is rigidly attached to one end of a brass tube, 
about 8 centimetres diameter, surrounding the spring and weight, 
and closed by a glass plate at the upper end of the incline, through 
which the weight is viewed. The tube is fixed to the hypotenuse 
of a right-angled triangle of sheet brass, of which one leg, inclined 
to it at an angle of about i radian, is approximately horizontal, and 
is supported by a transverse trunnion resting on fixed V-s under the 
lower end of the tube, and a micrometer screw under the short, 
approximately vertical, leg of the triangle. 
The observation consists in finding the number of turns and parts 
of a turn of the micrometer screw, required to bring the instrument 
from the position at which the bubble of the spirit-level is between 
its proper marks, to the position which equilibrates the spring-borne 
weight, with a mark upon it exactly in line with a chosen divisional 
line on a little scale of 20 half -millimetres, fixed in this tube in the 
vertical plane perpendicular to its length. 
The instrument is, as is to be expected, exceedingly sensitive to 
changes of temperature. An elevation of temperature of 1° C. 
diminishes the Young’s modulus of the German silver so much, 
that about a turn and a half of the micrometer screw (lowering the 
upper end of the tube at the rate of 2/3 millimetre per turn) pro- 
duced the requisite change of adjustment for the balanced position 
of the movable weight. About 1 J turn of the screw corresponds to 
a difference of 1/5000 in the force of gravity, and the sensibility of 
the instrument is amply valid for 1 /40 of this amount ; that is to 
say, for 1/200,000 difference in the force of gravity. Hence it is not 
want of sensibility in the instrument that can prevent its measuring 
differences of gravity to the 1/100,000 ; but to attain this degree of 
minuteness it will be necessary to know the temperature of the 
spring to within 1/20° C. I do not see that there can be any great 
