346 History of' Mechanical Inventions and 



that of powdered charcoal, brick, or soot, which may be used for painting 

 gates, palings, and other articles exposed to the air. 



4. Method of preventing the Fracture of Glass Chimneys. 



The glass chimneys which are now in such extensive use, not only for 

 oil lamps, but also for the burners of oil and coal-gas, very frequently 

 break, and not only expose to danger those who are near them, but occa- 

 sion very great expence and inconvenience, particularly to those who are 

 resident in the country. The bursting of these glasses very often arises 

 from knots in the glass where it is less perfectly annealed, and also from 

 an inequality of thickness at their lower end, which prevents them from 

 expanding uniformly by heat. The best method of detecting the knots is 

 to examine the glasses by polarized light, and reject those that exhibit at 

 the knots the depolarized tints. 



M. Cadet de Vaux {Bull, des Sc. Tech. Mars 1825. p. 180,) informs us, 

 that the evil arising from inequality of thickness may be cured by making 

 a cut with a diamond in the bottom of the tube, and he remarks that, in 

 establishments where six lamps are lighted every day, and where this pre- 

 caution was taken, there was not a single glass broken for nine years. 



5. Description of Griebel's Portable Night Clock 



This clock, constructed by M. Griebel of Paris, is represented in Plate 

 VI. Fig. 3 and Fig. 4, the former showing it in perspective, and the latter 

 in section. A is the globe which contains the clock movement and the 

 lamp B. The dial-plate C has a rim of ground glass with the hours paint- 

 ed upon it between E and C, Fig. 4. ; EE is a plate in the centre of the 

 ground glass ring to which the movement is fixed, F is a globe to protect 

 the wheel-work from dust. The rays of light BG, BG issuing from the 

 lamp B, illuminate the rim EC of the dial on which the hours and mi- 

 nutes are painted. Bull, des Sciences Technol. Jan. 1825, p. 40. 



It would, we think, be an improvement on this clock to place a mirror 

 between GG, to intercept all the rays that do not fall upon the rim of 

 ground glass, which, by means of another mirror behind B, would throw 

 some additional scattered light on the rim itself, while it woidd protect 

 the wheel-work from the direct radiation of the lamp- 



6. Description of M. Allard's Universal Bevel. 



This useful and ingenious bevel, which is represented in Plate VI. Fig. 

 2, is composed of two rules a, b, which open and shut round the joint a. 

 The shortest of these two b, carries a portion of a circle c, which passes 

 through a slit in d, where it may be fixed by a screw d. The other branch 

 a, is perforated with rectangular mortises in which are nuts, e, of the same 

 form, moving on a transverse pin f. in order to allow the screw g, which 

 passes through them, to incline itself and change its position as may be re- 

 quired- One of these screws is shown separately in Fig. 8- Near a is fixed a 

 flexible plate of steel hhh, which is jointed by caps it, and rivetted to the ends 



