14 
Dr. Fordyce's Account 
pansion of the screw A, and the frame EF, will carry down 
the point Q, so that it shall always be at an equal height with 
the point C, fig. 3. The points C and Q, fig. 3, 4, therefore, 
being always of the same height, may be considered as the 
points B C, fig. 1. The rod CD, fig. 1, is steel wire, the whole 
length of which weighs 13 grains. It is watch pendulum wire, 
No. 3. The point E is at R, fig. 4. 
SSSS, fig. 4, is the frame which connects the points I and 
E, fig. 1 ; to this frame the square brass bar I I is firmly fixed, 
so as to rise perpendicular from the frame SSSS ; on the side 
VV of this bar a semicylindric piece of crystal is attached, so 
that the cylindric part shall be towards R, and so that a num- 
ber of lines perpendicular to the horizon, tangents to this cy- 
linder, shall pass through the slit MM. 
WX is another square rod, with a crystal semicylinder also 
attached to the frame SSSS, so as to rise perpendicular to it, 
but sliding in the dove-tail groove-XY so as that it is capable 
of being brought nearer or farther from the cylinder attached 
to the rod I I. This may be done by a screw. It is clear then 
that the wire NO will pass between the cylindric parts of these 
two cylinders. 
If the screw Y was screwed so as to make the cylinders 
touch one another, all but the exact thickness of the wire NO, 
then it is clear that the under part of the wire, from V to O, 
would become flexible at V, but then the expansion of the 
tube I I, fig. 3, would not lower or raise the part of the wire 
VO so as to keep it always of an equal length ; but if the 
cylinders were opened the least farther, then the expansion or 
contraction of the tube 1 1, fig. 3, would raise or depress the 
part of the wire between the cylinder, and render the pendu- 
