354 History of Mechanical Inventions and 



In other respects the birds exercised all their functions, hearing and seeing, 

 eating and drinking as usual. 



M. Flourens repeated his experiments in presence of MM. Cuvier and 

 Dumeril, with the same results ; and however surprising and inexplicable 

 they may be, there seems no doubt of the facts as stated. — Revue Encyclo- 

 p^dique, Sept. 1828. Pp. 781—784. 



Art. XXVI.— history OF MECHANICAL INVENTIONS AND 



OF PKOCESSES AND MATERIALS USED IN THE FINE AND 



USEFUL ARTS. 



I. Description of a Differential Barometer, By the late W. Hyde 



WOLLASTON, M. D. F.R.S. 



This instrument is capable of measuring, with considerable accuracy, 

 extremely small differences of barometric pressure. It was originally con- 

 trived with the view of determining the force of ascent of heated air in 

 chimneys of different kinds ; but as its construction admits of any as- 

 signable degree of sensibility being given to it, it is susceptible of appli- 

 cation 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 syphon, with the legs parallel to 

 each other, is cemented at each of its open extremities into the bottom of a 

 separate 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 two or three inches ; while the remaining parts of the tube, to- 

 gether with the cistern, to the depth of about half an inch, are filled with 

 oil ; care being taken to bring the surfaces of water in both legs to the same 

 level, by equalising the pressure of the incumbent columns of oil. If the 

 horizontal pipe be applied to the key-hole of door, or any similar perfora- 

 tion in a partition between portions of the atmosphere in which the pres- 

 sures 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 that is elevated will just balance the external force 

 resulting from the inequality of atmospheric pressure upon the surface of 

 oil in both cisterns. This, however, is equal only to the difference between 

 the weight of the column of water pressing on one side, and that of an equal 

 column of oil which occupies the same length of tube on the other side ; 

 this difference depending 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 instru- 

 ment 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 the oil may be reduced to one-twentieth, one thirtieth, or any other 

 assignable proportion. The instrument may be converted into an areo- 

 meter, by closing both the cisterns, and by applying to the upper part of 

 each a trumpet-mouthed aperture, opening laterally. 



3 



