226 Prof. Oersted on the Law of the 



of a very strong glass tube glued into a hollow piece '% i, which 

 has a screw on its outer surface to screw into the opening in 

 the cover AC. In this cover there is another hole^?, which 

 is shut by a screw p E. TV is a wooden stand, surmounted 

 by a pillar RST, which serves as a support for the tube GH. 

 The two supplementary figures, Fig. 15 and 16, represent 

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

 of the apparatus. 



In order to make the experiment, the lid AC is unscrewed, 

 and the tube EF, containing air well dried, is put into its 

 place. The lid AC is then put on, and well closed up. The 

 tube GH is also put in its place, and, by means of a funnel 

 placed in the aperture^?, the cylinder is filled with water. The 

 pressure excited by this water is measured by the height of 

 the mercury in the tube GH. The apparatus is then closed 

 up by the screw, which goes into the aperture p, and when 

 mercury is poured into the tube GH, this fluid rises also 

 in the tube EF, and compresses the air which is con- 

 tained in it. The difference between the levels of the mer- 

 cury in EF, and in GH, gives us the compressing power, 

 and as the tubes have equal divisions, this difference of the 

 levels is found by a simple subtraction. Although the tube 

 EF of our apparatus is everywhere of a calibre nearly equal, 

 yet we have determined exactly the capacities corresponding 

 to the divisions by weighing the portions of mercury in each. 



The divisions on the tube GH reach only a few inches be- 

 low the cylinder. The other distances were measured by a 

 scale. 



In order to have the tube GH sufficiently long for 

 great pressures, we united, by iron screws, several tubes of 

 glass, each 7 feet in length. The experiment was always 

 made in a stair of the house, which contains the physical ca- 

 binet of the university, no apartment being sufficiently high 

 to permit the necessary elongation of the tube GH. 



TV ith this apparatus we have made several experiments, the 

 results of which are conformable to the law of Mariotte ; but 

 they have not all been attended with the same success, for it 

 is very difficult to make all the joints and screws sufficiently 

 impenetrable to mercury acting with high pressures. 



