363 



modulus of torsion in different woods, which he finds to vary from 

 about 9000 to 30,000 pounds, and to follow nearly the order of the 

 specific gravity. In the metals, the modulus of torsion is one sixteenth 

 of the modulus of elasticity. 



On a Differential Barometer. By the late William Hyde Wollaston, 

 M.D. F.R.S. Communicated by Henry Warburton, Esq. F.R.S. 

 Read February 5, 1829. [Phil. Trans. 1829, p. 133.] 



The instrument described in this paper is capable of measuring 

 with considerable accuracy extremely small differences of barometric 

 pressure. It was originally contrived with the view of determining 

 the force of ascent of heated air in chimneys of different kinds ; but 

 as its construction admits of any assignable degree of sensibility being 

 given to it, it is susceptible of application to many other purposes of 

 more extensive utility. A glass tube, of which the internal diameter 

 is at least a quarter of an inch, being bent in the middle into the 

 form of an inverted siphon, with the legs parallel to each other, is 

 cemented at each of its open extremities into ,the bottom of a sepa- 

 rate cistern about two inches in diameter. One of these cisterns is 

 closed on all sides, excepting where a small horizontal pipe opens 

 from it laterally at its upper part ; while the other cistern remains 

 open. The lower portion of the glass tube is filled with water, or 

 other fluid, to the height of wo or three inches ; while the remain- 

 ing parts of the tube, together with the cistern, to the depth of about 

 half an inch, are filled with oil ; care being taken to bring the sur- 

 faces of water in both legs to the same level, by equalizing the pres- 

 sures of the incumbent columns of oil. If the horizontal pipe be 

 applied to the key-hole of a door, or any similar perforation in a par- 

 tition, between portions of the atmosphere in which the pressures 

 are unequal, the fluid in the corresponding half of the instrument will 

 be depressed, while it is raised in the opposite one, until the excess 

 of weight in the column thus elevated will just balance the external 

 force resulting from the inequality of atmospheric pressures upon 

 the surfaces of oil in both cisterns. This excess, however, is equal 

 only to the difference between the y weight of the column of water 

 pressing on one side, and that of an equal column of oil which occu- 

 pies the same length of tube on the other side. This difference de- 

 pending upon the relative specific gravities of the two fluids will, in 

 the case of olive oil and water, be about one eleventh of the weight 

 of the column of water elevated ; but the sensibility of the instrument 

 might be increased at pleasure, by mixing with the water a greater 

 or less quantity of alcohol, by which the excess of its specific gravity 

 over that of oil may be reduced to one twentieth, one thirtieth, or 

 any other assignable proportion. The instrument may be converted 

 into an anemometer by closing both the cisterns, and by applying to 

 tlie upper part of each a trumpet-mouthed aperture opening laterally. 



