8 
Dr. Fordyce's Account 
ing a pendulum on these principles, either of the fixed points, 
according to the circumstances, I A or H be placed too high or 
too low, then the pendulum will vary in its length, and of 
consequence swing different times in different degrees of heat. 
That is to say, suppose the case be taken where the rod 
BG is made of materials^ which expand more than CD, if in- 
stead of rendering I the fixed point, it is made G, nearer to B. 
then the point C will not be raised sufficiently above the point 
E, the pendulum will become longer by heat, and make fewer 
vibrations in a given time. But if the fixed point in this case 
be brought lower than I, to the point L, then the point C 
will be raised higher when the whole is expanded by heat 
from the point E, ED will be rendered shorter, more vibra- 
tions will be performed in a given time, and e contra . There- 
fore if there be a power in the apparatus of altering the fixed 
point I, if found too high or too low by experiment, we shall 
be able to find out a true point, and make an adjustment ac- 
cordingly. That is, if heat occasions a pendulum to make 
fewer vibrations, the fixed point is too high ; if, on the con- 
trary, it makes too many vibrations, then the fixed point will 
be too low. 
I come now to show how these principles may be applied 
in structure. 
As it is more convenient in practice to have the fixed point 
higher than A, which is equal in height to D the bottom of 
the pendulum, a substance should be chosen for the rod AB 
which expands and contracts more by heat and cold than the 
matter of which the rod CD consists, so that the fixed point 
should be at G, if the materials connecting A and E were in- 
capable of being expanded or contracted by heat or cold : 
