NATUilAL PHILOSOPHY. 171 



other non- conducting substance. An electric current is then established, 

 so that when the feeler rests on the metallic surface it passes through the 

 coils of the magnet, and causes it to lift the graver from the plate to be 

 engraved. As soon as the feeler reaches the drawing and passes over the 

 non-conducting ink, the current of electricity is broken, and the magnet 

 ceases to act, and by a self-acting mechanical arrangement the current is 

 at the same time diverted through the coils of the second magnet, which 

 then acts powerfully and presses the graver down. This operation being 

 repeated until the feeler has passed in parallel lines over the whole of the 

 drawing, a plate is obtained engraved to a uniform depth, with a fac-simile 

 of the drawing. From this a type-metal cast is taken, which, being a 

 reverse in all respects of the engraved plate, is at once fitted for- use as a 

 block for surface printing. The machine is the invention of Mr. "NYilliam 

 Hausen, of Gotha. Journal of the Society of Arts. 



SPECIFIC HEAT OF GASES. 



M. Regnault has presented to the Academy a very long memoir on the 

 Specific Heat of Gases under constant pressure and variable volume, and 

 under constant volume and variable pressure. After detailing the history 

 of this important question, M. Eegnault explained in a brilliant lecture 

 his method of observation, the arrangement of his apparatus, and the 

 important results he had obtained results which entirely change the pres- 

 ent state of the science, being in complete discordance with the theory of 

 Laplace and Poisson, and with the observations of Desormes, Gay-Lussac 

 and Dulong. 



It has heretofore been admitted, that the capacity for heat under constant 

 pressure is always greater than that under constant volume; and the 

 ratio of these capacities is equal to unity plus a fraction, which, in air, is 

 338,000 according to Dulong, 375,000 according to Gay-Lussac, 421,000 

 according to Poisson, &c., &c. By operating in an entirely new mode, 

 and under conditions that he thinks better, M. E,egnault seems to have 

 shown that the difference between these is nothing, or infinitely small. 



Conceive two concentric globular vessels, one whose capacity is a litre 

 filled with gas, air for example, under a pressure of ten atmospheres, the 

 other with a capacity of ten litres. This system of two vessels is immersed 

 in a water bath kept at a constant temperature, if, after having made a 

 vacuum in the second globe, we allow the air to enter it from the first, so 

 that it now occupies a bulk ten times greater, there is neither elevation nor 

 depression of the temperature. There will be, however, a depression of 

 the temperature, if, at the same time that the air enters the larger globe, a 

 small quantity is allowed to pass out by an- orifice in the globe ; and the 

 amount of depression of the temperature is constantly proportional to the 

 mass of gas which has escaped into the atmosphere. If the air which 

 escapes is made to do work, as for instance to move a turbine, re-action 

 wheel, or pump, the cooling increases in proportion to the work done ; 



