360 Historic of Mechanical Inventions and 



of temperature in spring and summer ; and the refrigeration 

 of 18° in both cases is no very extreme supposition. * 



I forbear to enlarge farther upon a topic so confessedly ob- 

 scure, and which, I conceive, may at some future time be re- 

 duced to practical accuracy. For the present, I content my- 

 self with referring to the paper on the much more unaccount- 

 able phenomena of the Monte Testaccit), the details of which 

 will, I think, satisfy the reader that, by the hypothesis there 

 supported, all other cases on record will easily be solved, in- 

 cluding the " Grotta del Vento" in Ischia. 



Art. XXIIL— history OF MECHANICAL INVENTIONS AND 



o:f processes and materials used in the fine and 

 useful arts. 



1. Notice of the Rock Crystal Watch of M. Rebillier. From 

 a Report to the Institute, by MM. Proky and Navier. 



M. Rebillier does not propose to introduce any improve- 

 ment into the art of watch-making ; but his work is dis- 

 tinguished by the nature of the materials employed, and by 

 the difficulty, the delicacy, and the perfection of the work- 

 manship. This watch, whose dimensions are so small that it 

 may be worn round a lady's neck, is almost entirely executed 

 in rock crystal. The transparency, of the substance allows us 

 to see the interior mechanism of it. The two toothed wheels 

 which conduct the hands are in rock crystal; the other wheels 

 are in metal, to prevent the accidents which would arise from 

 the breaking of the main spring. All the screws are cut in 

 the crystal, and all the pivots turn in ruby holes. The bridge 

 and the piece which form the escapement are in sapphire, the 

 balance is in rock crystal, and the spiral spring in gold. The 

 author ascribes to the feeble* dilatation of these two sub- 

 stances the regularity of the motion of the watch ; f but this 



• Humboldt, in describing the Peak of TenerifFe, has made some re- 

 marks on the theory of ice caves, but they are less applicable to merely 

 cool currents of air. — Personal Narrative, L 154. 



t In November 1824, Dr Brewster proposed to construct the balances of 



