336 PHILOSOPHICAL TRANSACTIONS. [ANNO 1/94. 



//. 0/ a New Pendulum. By George Fordyce, M. D., F. R. S. ; being the 



Bakerian Lecture, p. 2. 



Let ab and CD, fig 1 , pi. 4, be 2 rods of any solid of the same kind, and of a simple 

 or uniform texture. Let these 2 rods be exactly of the same length ; let them 

 be connected at the top with a rod bc, which is perfectly inflexible ; and let the 

 angles abc and dcb be both right angles, so that ab and do shall be parallel to 

 each other, and in the same plane ; let the rod ab be fixed at the point a, and 

 perpendicular to the horizon : then the rod cd shall likewise be perpendicular to it, 

 .excepting for the curvature of the earth between b and c, which in a foot or 2 

 may be considered as nothing : let the rod CD be loose at the end d, so as to be 

 capable of rising up or falling down ; in this case, if heat be applied equally to 

 both rods, so as to expand them both, and lengthen them, the rod ab will raise 

 up the rod bc, and lift up the rod dc ; but the rod dc being equally lengthened 

 by heat with the rod ab, the point d will be brought downwards by the length- 

 ening of the rod dc, as much as the point c is raised by the lengthening of the 

 rod ab by the heat. In consequence the rod dc will have its end d in a line 

 exactly parallel to the horizon, and cutting the end of the rod ab at a, as it did 

 before the heat was applied. And the same thing will be true if the rods ab, 

 DC, be shortened by exposure to cold: so that in all cases of heat or cold the end 

 of the rod cd at d, and the end of the rod ba at a, shall be in a line parallel to 

 the horizon. 



Take a point e near any part of the rod cd, and let that point e be connected 

 with the point a, where the end A of the rod ab is fixed, and let the matter 

 which unites them be perfectly inflexible, and incapable of being altered by heat: 

 then the part of the rod cd, intercepted at the point e, and forming ed, will 

 always be of the same length whether the temperature of heat is greater or less. 

 For supposing a point f be taken near the rod ab, and of the same perpendicular 

 height with e, so that a line drawn from e to f shall be parallel to the horizon; 

 if the part of the rod ab, o[)posite to f, should rise up in consequence of being 

 expanded by heat above f, it will carry up the point of the rod dc, which was 

 opposite to E, to an equal height with itself, and therefore would cut off from 

 the length of that part of the rod which was formerly opposite to e, a length 

 equal to that which was added to what was formerly ed, by the heat. Tliere- 

 fore the point opposite to e, in the rod dc, will form ed, which will always con- 

 tinue of the same length, if the heat be increased : and by similar reasoning, it 

 will likewise continue of the same length if the part of the rod ab af is shortened 

 by cold : therefore the part of the rod dc cut off by the point e, so as to form 

 ED, will always be of an equal length, and the point D will always be of an equal 

 height. 



At the point e let there be an apparatus which will render that part of the rod 



