DR. WOLLASTON ON A DIFFERENTIAL BAROMETER. 
135 
Into a vessel so constructed, a small quantity of Avater 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 x x T th 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, Avere 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 ^th, xu 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. 
Captain Flinders informs us that on the coast of New Holland during the 
