'290 



annexed, for the purpose of measuring pressure ; and is so adjusted, 

 that the number of pounds pressing on its piston indicates directly 

 the number of atmospheres used for compression. The author next 

 describes the piezometer which he employed in measuring the com- 

 pression valve, consisting of a tube of water inserted in quicksilver, 

 the contracted part of which tube contained a small steel disk, hav- 

 ing a hair spring attached to it to keep it in its position when pressed 

 up. The piezometer, properly arranged, was then placed in the re- 

 ceiver of the compressor, filled with water at 50, the pump screwed 

 into its place ; and as soon as the intended pressure had been effected, 

 the piezometer was examined, and the indicating spring was always 

 found more or less raised in the tube, according to the power em- 

 ployed. Mr. Perkins states that water, under a pressure of 2000 

 atmospheres, compressed in a tube eight inches long, was diminished 

 one twelfth of its length ; and has annexed to his paper a table, show- 

 ing in inches and parts the compression of a column of 190 inches of 

 water, corresponding to every 10 atmospheres to 1000 inclusive. 



The author found that acetic acid crystallized under a pressure of 

 1100 atmospheres ; that under a pressure of 500 atmospheres water 

 took up its volume (?) of air, none of which was again given out on 

 removing the pressure ; that air, under a pressure of 1 200 atmo- 

 spheres, became a limpid liquid (permanent ?) ; and that carburetted 

 hydrogen was entirely liquefied under the same pressure. 



On the Figure of the Earth. By George Biddell Airy, M.A. Fellow 

 of Trinity College, Cambridge. Communicated by J. F.W. Herschel, 

 Esq. Sec. R.S. Read June 15, 1826. [Phil. Trans. 1826, Part III. 

 p. 548.] 



Account of Experiments made with an Invariable Pendulum at the 

 Royal Observatory at Greenwich, and at Port Bowen, on the eastern 

 side of Prince Regent's Inlet. By Lieutenant Henry Foster, R.N. 

 F.R.S. Read April 6, 1826. [Phil. Trans. 1826, Part IV. p. 1.] 



The author, in the prefatory introduction to this paper, states first 

 of all his own previous experiments made by Captain Hall and him- 

 self, on the South American station ; he then describes minutely the 

 nature of the experiments contained in this communication ; the in- 

 struments employed ; the precautions used ; and the method pursued 

 in calculating their results. The experiments comprise three distinct 

 series. The first made at the Royal Observatory at Greenwich, be- 

 fore his departure with Captain Parry for the North Western Expe- 

 dition, in which it will suffice to remark, that all the adjustments 

 and precautions recommended by Captain Kater in his paper of 1819 

 were strictly adhered to. In addition to which, observations were 

 made, not only of the disappearances of one pendulum behind the 

 other, but also of its reappearances, as recommended by Captain 

 Sabine ; methods which, he observes, as far as the deduction of the 

 acceleration of the pendulum when compared at different stations 



