82 SECTION PHYSICS. 



authentic. As you may see, Pascal wrote on it with his own hand this 

 inscription : 



" Esto probati symbolum hoc. 



" Blasius Pascal Arvernus, inventor. 20 Mai, 1652." 

 How much it is to be regretted that the original instruments employed 

 to prove and to measure the effects of atmospheric pressure should 

 not likewise have been handed down to us. Let me also draw your 

 attention to these two globes with clock movement by Just Burg (1586) 

 and Jean Reinhold (1588), and which are, moreover, so remarkable for 

 their chasing, attributed to Jean Goujon. The names of the makers 

 are, no doubt, of less importance, but the inscriptions are contemporary 

 with the change in the calendar, and the mechanism within them 

 cannot but offer us curious problems which demand no little investiga- 

 tion to prove certain much controverted points in the history of clock- 

 making 



On the other hand, our collection, from the last years of the i8th 

 century, is extremely rich, in spite of frequent gaps, and you may see 

 the proof of it in the number of instruments which I have placed 

 before you, and which should you find the subject interesting we 

 will examine in the order which has been so suitably chosen for the 

 classification of the objects sent to this exhibition. 



But before entering into these divisions, let me call to your notice 

 this small cathetometer by Dulong, the first that was ever constructed, 

 and which was made under the direction of that celebrated man. It 

 has been used as a model for all similar instruments, by means of 

 which the numerical values of the differences observed in the principal 

 phenomena have been able to be computed with exactness. 



In the Section of Sound we should have wished to renew Savart's 

 beautiful experiments on vibrations, but the plates on which he studied 

 them have not been able to be arranged as they were originall>, and 

 so we have been forced to be satisfied with his musical instruments, 

 near which you see the Register of Duhamel, who was the first to 

 succeed in inscribing with sufficient precision the vibrations of 

 sound. The determination of the velocity of sound is brought before 

 you by M. Regnault's original apparatus and also by the one of M. Le 

 Roux, who arrived, at about the same period, at a nearly identical 

 conclusion. 



