Prof. Oersted on the Compression of Gases. 103 



Captain Schwendsen and myself, desirous of making some 

 experiments on the air-gun, felt the necessity of first trying the 

 extent of the Mariottic law, to serve us for a basis. The ap- 

 paratus generally used for this purpose is known to consist of 

 a curved tube ABCD (PI. II. fig. 1.) one part of which DE 

 contains the air, and the other ABCE mercury; the object 

 of which is to inclose and compress the air. This apparatus, 

 however, has several inconveniences : it is difficult to subdivide 

 the part DE of the tube into equal spaces ; besides, this part 

 is extended by the pressure within, and there is some risk 

 of its bursting if the pressure becomes too great. To coun- 

 teract such an accident, tubes of a smaller diameter are used ; 

 but this expedient again creates a friction sufficient to disturb 

 the results to a considerable degree. To avoid, therefore, all 

 these inconveniences, we had recourse to an apparatus which, 

 constructed after the same principle, had formed part of my 

 apparatus for the compression of water. Fig. 2. presents a ver- 

 tical section of this new apparatus. ABCD is a very strong 

 glass cylinder, having a brass lid. EF is a graduated glass 

 tube supported by an iron frame I m n o, which at its lower 

 end terminates in an iron cup containing some mercury. 

 This closes the tube EF before it is plunged into the mass of 

 mercury spread at the bottom of the cylinder. IK shows the 

 superior limit of the quicksilver. GH represents a part of a 

 very strong glass tube, which is cemented into a hollowed 

 piece of metal, part of the outer surface of which is adapted 

 to a nut which is in the lid of the cylinder. The two addi~ 

 tional figures marked by a and b in fig. 2. represent, one the 

 frame Imno, and the other the transverse section of the lower 

 part of the apparatus. If an experiment is to be made with 

 this apparatus, the lid AC is screwed off; the tube EF, filled 

 with carefully dried air, is plunged into the cylinder, the lid 

 screwed on again, and well secured. Then the tube GH is 

 also put in its place, and the cylinder is filled with water by 

 means of a funnel placed in the aperture P. The pressure 

 produced by this is measured by the rising of the mercury in 

 the tube GH. Finally, the apparatus is closed by means of 

 the screw fitted to the aperture P, and mercury poured into 

 the tube GH, which also rises in the tube EF, and compresses 

 the air contained in it. The distance of the levels of the mer- 

 cury in the tubes EF and GH, both being graduated alike, 

 show by simple subtraction the magnitude of the compressing 

 power. The tube EF is nearly throughout of equal size ; never- 

 theless we have determined the spaces corresponding with the 

 divisions by means of quantities of mercury carefully weighed. 

 The graduation of the tube GH extends only a few inches 



above 



