23G Proceedings of Philosophical Societies, [March^ 



by tin, must have been formed by a method similar to that at 

 present known by the name of cliche (by letting a heavy weight 

 fall on them). He mentions, likewise, other processes^ which 

 show the part he took in the progress of the art of multiplying 

 copies. 



in a second memoir he has explained the construction of a 

 prismatic micrometer, for measuring exactly the diameters of the 

 sun and moon. The object glass of this niicrometer is composed 

 of rock crystal and flint glass; it is at the same time achromatic, 

 and gives a refraction of 26'^. This is not sufficient for mea- 

 suring, which would require a refraction at least of 32'. But an 

 achromatic prism oi' crystal, similar to that in ordinary micro- 

 meters, moving along tiie axis, serves to complete the measure- 

 ment, though it has only a double refraction of 6', because we 

 may choose a time when the apparent diameters are only 30' or 

 31'. The advantage of this construction would be that in a glass 

 of this nature, supposing only 2 metres (78*742 inches) of focal 

 length, each second will occupy the space of 3 miUimetres 

 (0*11811 inch). Hence it will not be difficult to estimate 

 the tenth of a second. Before this improvement of his micro- 

 meter, M. Rochon had measured the diameters of the smallest 

 planets ; and from the calculation of hi& dbservations we have in 

 general found that something ought to be subtracted from the 

 diameters commonly adopted by astronomers. It will be curious 

 to see whether from the measurements which M. Rochon will 

 take of the sun and moon with his new instrument, we shall 

 have a similar diminution to make in the diameter of these 

 luminaries, which are of much greater importance for practical 

 astroiiomy, and which in fact have always been found smaller by 

 some seconds in proportion as the instruments became more 

 perfect. 



In a third memoir M. Rochon has given the general theory of 

 instruments serving to measure angles, either by mirrors, or by 

 achromatic prisms of glass or rock crystal. 



The fourth memoir contains an account of the employment of 

 metallic wire-work to render buildings incombustible. He 

 relates an experiment made by M. Dyle, who has covered with 

 his cement, impenetrable to water, six metres square of this 

 wire -work. 



In the fifth memoir, on printing, after having spoken of the 

 origin and progress of this art, he points out the method of 

 making a small number of characters serve for printing a large 

 work, particularly for printing tables of logarithms. 



The sixth memoir was destined to point out the advantages of 

 employing mica, commonly called Muscovy glass, for letting in 

 the light. 



On the 22d of June M. Rochon presented to the Class an 



