28 THE APPLICATIONS OF PHYSICAL FORCES. [BOOK i. 



This result of theory was confirmed at Paris in 1851, under the 

 dome of the Pantheon, by Leon Foucault. This distinguished phy- 

 sicist arranged his experiment, which attracted a number of curious 

 people, in the following way : At the highest point of the interior 

 of the dome a steel wire about 64 metres in length was firmly fixed 

 into a metal plate ; this carried at its extreme end a very heavy 

 brass ball. When removed from its vertical position and left 

 to itself, this pendulum very slowly executed a series of oscilla- 

 tions in a plane which theory, as we have before stated, proves to be 

 invariable in space. On the hypothesis of the earth being stationary 

 the orientation first given to this plane would therefore have been 

 kept. Now, the numerous spectators of this curious experiment were 

 able to observe a deviation. 



In one hour, the arc measuring this deviation was very nearly that 

 indicated by theory, namely 11 17' 39". Two little mounds of sand 

 placed on a circular balustrade and at the extremities of the same 

 diameter, were by degrees cut through in opposite directions by a 

 metal point fixed below the ball of the pendulum, so that the ap- 

 parent deviation of the plane of the oscillations, due to the rotation 

 of our globe, and therefore this rotation itself, were rendered per- 

 ceptible to the eyes of all. 1 



Y. BALANCES USED IN COMMERCE OR TN THE ARTS. 



We have described the balance of precision in the first book of 

 the Forces of Nature; it is the only one used for scientific deter- 

 minations of weight requiring great accuracy. Other kinds of 

 balances, more roughly constructed and intended for near approxima- 

 tions, are used in commerce and industry ; we will hastily describe those 

 most used, without entering into the details of their construction. 



The Eoman steelyard, called in France Romaine, is the one most 

 anciently known : its French name is taken from the ancient Romans, 

 by whom it was used. Its construction is very simple, and is based 

 on the principle of mechanics that the weights of two heavy bodies 



1 Leon Foucault has demonstrated the rotation of the earth in another way, by 

 a similar mechanical principle. The instrument to which we allude is the gyroscope. 

 The reader will find the description of it in the most recent treatises on Mechanics. 



