and Astronomical Instrument Makers at Paris. 257 



in all the optical manufactories of Paris; and remarked to 

 me, that at Benedictbeurn no one would take the trouble to 

 work glass with the hands, in the way in which it was done at 

 Paris. 



M. Kutsch is one of the best workmen for common mea- 

 suring instruments. He lives at No. 41, Rue des Lombards. 



Many of these particulars I owe to some interesting con- 

 versations which I had with M. Arago, and they are but a 

 slight specimen of the kindness which I have received from 

 him during my residence in Paris. But it is not myself only 

 who am indebted to him ; and I consider it my duty, before 

 I leave France, to apprize you of your obligations to him for 

 the care and assiduity with which he has compared and ex- 

 amined the instruments intended for you, as well as for the 

 time he has devoted to them at a period when he was more 

 than usually occupied by the exhibition of the products of 

 French industry, of which he is one of the examiners. So 

 that this cannot be attributed solely to the interest which he 

 takes in every thing which relates to the sciences, but also 

 to his desire to serve you, and to evince his friendship for 

 you. 



M. Pecqueur, the supermtendant at the Conservatoire des 

 arts et metiers, showed me a clock which exhibited with great 

 accuracy sidereal time and mean solar time. He has con- 

 trived this by means of a mechanical invention, which en- 

 ables him to give the same force to two different movements 

 of the clock : for instance, to two wheels, whose movements 

 are to each other in any given ratio. The pendulum of this 

 clock is compensated by mercury contained in the annular 

 space formed by two concentric cylinders, the inner one of 

 which is made of iron, and the outer one of glass : and, as the 

 expansion of iron by heat exceeds that of glass, this serves to 

 augment the compensation already produced by the expan- 

 sion of the mercury. The cylinder of iron is hollow, and 

 open at both ends ; and consequently enables the air as much 

 as possible to affect the temperature of the mercury. The 

 rod of the pendulum is made of steel. M. Pecqueur has not 

 yet fixed the price of this pendulum ; at least, I have asked 

 him for it in vain. He has also shown me a similar contri- 

 vance for pocket chronometers. The movements of the watch 

 being attached to this piece of mechanism, the watch will in- 

 fallibly adapt itself to the motion required. This effect is 

 produced by a spring which acts upon the spiral. 



This invention was presented to the Institute some years 

 since; but no report had been made of it, when a letter of 

 M. Pecqueur induced the President to call for this report in 



Vol. 63. No. 312. April 1824. Kk the 



