DR. WOLLASTON ON A DIFFERENTIAL BAROMETER. 135 



Into a vessel so constructed, a small quantity of water is first poured, so as 

 to occupy two or three inches of the lower part of the glass tube. Equal 

 measures of oil are then poured into each cistern, so as to fill the upper part of 

 both legs of the tube, and also to rise in each cistern to the depth of about half 

 an inch. 



When the two surfaces of the water in the two legs are seen to be on the 

 same level, or have been rendered so, by equalizing the pressures of the incum- 

 bent columns of oil, the instrument is adjusted ready for use. 



If the horizontal pipe from the closed cistern be now applied to the key-hole 

 of a door or to any other perforation, through which air may enter by excess 

 of external pressure, the pressure applied to the surface of the oil in that 

 cistern will lower the water in the corresponding leg, and raise it in the 

 opposite one, until the excess of the weight thus elevated is sufficient to ba- 

 lance the force by which the pressure of the external air exceeds that within 

 the chamber. 



It is not, however, the entire excess of the longer column of water which in 

 this case acts as an equipoise ; since that excess is counteracted by an equal 

 elongation of the column of oil on the side depressed ; so that the pressure ex- 

 erted is only the difference between the column of water and an equal column 

 of oil ; which, in the case of olive oil, amounts to about xVth part of the appa- 

 rent elevation. In this case therefore the variations of this instrument are 

 about eleven times as great as they would be, were water alone employed. 



If for any other purpose an instrument of greater sensibility be required, the 

 scale of its variations may be enlarged at pleasure, by mixing a greater or less 

 quantity of alcohol with the water, until the excess of its specific gravity above 

 that of oil is reduced to ^Vth, ^^th, or in any other proportion ; so that finally 

 the spirit being of the strength called proof (which appears originally to have 

 been named from this test), will rest with steadiness in no position, or being 

 still further attenuated, will rise and suffer the oil to subside in the tube. 



By a slight variation in the form of this instrument ; that is by closing both 

 the cisterns, and by applying to the upper part of each a trumpet-mouthed 

 aperture, opening laterally ; it may be made to serve the purpose of an Ane- 

 mometer, ^^ffl 



Captain Fi^inders informs us that on the coast of New Holland during the 



