4 
Dr. Fordyce's Account 
ing DE, may become a pendulum. Thus we shall procure a 
pendulum of the same length, whatever be the degree of heat. 
Let the rod AB and the rod DC be of different species of 
matter, so that the rod AB shall be lengthened by being heated 
to the same degree, more than the rod DC ; then, if they be 
both of the same length, heat would carry up the end of the 
rod CD, at D, higher than the fixed point A ; but if a part be 
cut off from AB at G, so that the whole of the expansion of the 
remaining part GB, shall be equal to the whole of the expan- 
sion of the whole rod DC, and that in every increase of heat, 
then the same thing would happen ; and the part of the rod DC, 
cut off by the apparatus at E, would always remain of the same 
length. If, therefore, it is wished to render DE always equal 
in length, the fixed point A must be brought nearer to B, so 
as to shorten the rod AB, that is at G, so that the w T hole of 
the expansion of GB by heat, shall be equal to the whole of 
the expansion of DC by the same degree of heat. 
Hitherto I have supposed that the substance which con- 
nected the points A and E was incapable of being expanded 
or contracted by heat : but no such substance is to be found. 
I shall now suppose that the substance which connects the 
points A and E is capable of being expanded by heat. If it 
was capable of expansion equal to the matter of which the rods 
AB and CD consist, then it is evident that no advantage 
could be gained so as to render the part of the rod CD opposite 
to the point E down to D always equal. But it is clear that 
the expansion of AE, supposing the point A a fixed one, 
would carry the point E higher up towards C, if the heat 
was greater, so that ED would by this means be rendered 
longer ; and the contraction of AE, when exposed to a greater 
