Intelligence and Miscellaneous Articles, 147 



fastened on a cast-iron stand. A piston, also of steel, is moved in this 

 cylinder by a square-threaded screw, which works in a female screw 

 of bronze, wedged in the axis of a fly, also of cast iron. When this 

 fly is turned by means of the pegs on its circumference (as the screw 

 cannot follow it in its rotation, owing to a catch secured by two 

 slide bars) , the piston traverses the vacuum of the cylinder in a direc- 

 tion determined by the direction of the motion of the fly. The water 

 which the cylinder contains cannot escape ; a leather is fitted so per- 

 fectly that, even under pressures of more than 800 atmospheres, 

 scarcely a drop of liquid escapes. To the cylinder in which the 

 compression is effected a steel laboratory-tube can be united by a 

 capillary tube of copper — which, leaving that part of the apparatus 

 quite free, allows the majority of the experiments to be made here. 

 The pressure is estimated by two mutually controlling processes : 

 (1) by a lever which rests on a very moveable valve ; (2) by a Des- 

 goffe's modified manometer, which I will briefly describe. 



This instrument consists of a cylindrical cast-iron vessel, filled 

 with mercury, upon which rests a metallic disk. A thin membrane 

 of caoutchouc separates the disk from the mercury, which con- 

 sequently cannot escape. A metal rod penetrates to the centre of 

 the disk, passing through a leather fixed in a bronze cylinder con- 

 nected with the pressure-machine. When the compressed water 

 acts on the small piston, the pressure is transmitted to the mercury, 

 ■which rises in a vertical glass tube, communicating with the reser- 

 voir. 



If the ratio of the surface of the small piston to that of the disk is 

 = 1 : 100, then for a pressure of 100 atmospheres the mercury will 

 only rise in the manometric tube 1 atmosphere, or 0*76 metre. 



A grave a priori objection might be made to this apparatus ; in, 

 fact, it is not known what resistance the leather exercises on the 

 piston. In the apparatus employed by me, the ratio of the surfaces 

 is =1 : 2 1 2, and it is sufficient to lower the piston 1 of a millimetre in 

 order to raise the mercury 4*30 metres, the height of my manometric 

 tube. The path traversed being very small, the resistance will be 

 nearly none. To overcome the inertia, the mercury is caused to oscil- 

 late about its position of equilibrium in the glass tube by means of a 

 small lever, which acts on the compressing disk. The manometer 

 thus constructed has been verified up to 80 atmospheres by the help 

 of a very large manometer, in which the compressed air was replaced 

 by hydrogen. The graduation was based on the numbers published 

 by M. Regnault. The apparatus for pressure, such as I have just 

 described it, easily gives pressures from 8 to 900 atmospheres, which 

 can be maintained for a considerable time. Danger from the burst- 

 ing of any part of the machine, there is almost none : steel tubes 

 filled with liquid have frequently split without any of their parts 

 being projected. 



In an experiment, in which I subjected to about 850 atmospheres 

 pressure 60 cubic centimetres of hydrogen, the laboratory-tube was 

 broken, the compressed gas suddenly expanded and exploded with 



L2 



