392 THE CONNECTION AND EQUIVALENCE OF FORCES. 



an equal force must be exerted by the rod in its extension 

 as in its contraction. Now, experiment has shown that the 

 relation expressed by the numbers above given, must exist 

 between a given extension of a bar of iron, and the heat or 

 weight which has caused that extension, viz., that a quantity 

 of heat sufficient to raise a pound of water one degree in 

 temperature, will, when communicated to a bar of iron, en- 

 able it to elevate a weight of 1,350 Ibs. to the height of one 

 foot. 



An interesting application of this fact was long ago made 

 in the Conservatoire des Arts et Metiers, in Paris. In this 

 building, which was formerly a convent, the nave of the church 

 was converted into a museum for industrial products, machines, 

 and implements. In its arch, traversing its length, appeared 

 a crack, which gradually increased to the width of several 

 inches, and permitted the passage of rain and snow. The 

 opening could easily have been closed by stone and lime, but 

 the yielding of the side walls would not have been prevented 

 by these means. The whole building was on the point of 

 being pulled down, when a natural philosopher proposed the 

 following plan, by which the object was accomplished. A 

 number of strong iron rods were firmly fixed at one end to a 

 side wall of the nave, and after passing through the opposite 

 wall were provided on the outside with large nuts, which 

 were screwed up tightly to the wall. By applying burning 

 straw to the rods, they extended in length. The nuts by this 

 extension being now removed several inches from the wall, 

 were again screwed tight to it. The rods on cooling con- 

 tracted with enormous force, and made the side walls ap- 

 proach each other. By repeating the operation the crack 

 entirely disappeared. This building with its retaining rods 

 is still in existence. 



The working power of a machine, set in motion by elec- 

 tricity, can be expressed by numbers, in the same way as the 

 mechanical effect of heat. An electrical current is generated 



