Hydrostatics.

279

that the lightejl Body P may be depress'd in theheavieji Fluid, by any Contrivance to keepthe said Fluid from pressing on the underSurface of the light Body, by whichMeans only light Bodies are made to swim.Thus Cork or Wood will abide at the Bottomof a Vessel fill’d with Quick-silver.

Again: On the other Hand , the heaviejiBody M may be made to swim in the lightejlFluid , by keeping the said Fluid from pressingon its upper Surface , by means of the TubeNO: For when by this means it is im-mersed so deep as to keep off an equalWeight of the Fluid, the Pressure then ofthe Fluid acting upon its under Surface up-wards will be equal to the Weight of theSolid tending downwards; and therefore ifthe Solid be funk ever so little deeper, it must

swim

mage at C, having so much Water put into its Bodythat when the Hole in its Foot is sealed up, it shall bejust light enough to swim in the said salt Water;Things being thus prepared ; if so much hot freshWater be put into the Jar AB as will fill it to the Top,the Consequence will be, that the Fluid being nowlighter, will not sustain the Image C which will fink tothe Bottom ; and the Heat of the Water will rarify theAir in the Bubble B, by which Means the Water in itwill be in Part extruded, and the Bubble therebybecoming lighter, will rife to the Top. Thus by thisArtifice, the Image and Bubble will spontaneously (asit were) change Places ; which pleasant Experiment,or Hydrostatic Problem, was first proposed by Mr. fohnCafwell, Astronomy Professor at Oxford, near 40 Yearsago. ' T 4