Joty—Useful Methods of Teaching Elementary Physics. 217 
up, the water will not help any further or rise any further, and 
we may pull up the piston as high and dry as we please, 
supporting then the full atmospheric pressure upon the piston 
unaided. 
Boyle’s Law.—Take the glass tube, as previously described ; 
drop in the piston, with the float f upwards, towards the open 
end. Let it sink till about half way down; then pour down a 
little mercury, and raise the tube upright. A certain volume of 
air is enclosed below the piston and mercury. Fix the tube by a 
clip on a retort-stand. Hold up a scale beside it, and read the 
length V, of the column of air, also the length m, of the mercury. 
Write these down. Add to m,—with an obvious explanation— 
the length of a column of mercury equal to the barometric length 
(or the length of the just-sinking hanging column), and write up 
the added lengths as P,, under the number Y;. Multiply these 
together. 
Now pour mercury down the tube till m, has increased to 50 
or 60 cm. Measure this length. Add, as before, the barometric 
length, writing this up as P,. Then read V, by the scale, and 
multiply V2 by P2, and show that P, x V2 is (closely) equal to 
iP xe Vy. 
In carrying out this experiment, it is necessary to guard against 
handling the lower part of the tube, or temperature errors will 
arise. It is also requisite to leave the volume V, a few moments 
to assume the temperature of the air and part with the heat gene- 
rated in it by compression. Very accurate measurements may be 
made if the lower end is cooled in melting ice during experiment, 
and more advanced students might be asked to plot successive 
volumes and pressures. Such might be required also to determine 
accurately the successive volumes by weighing the tube afterwards 
when containing mercury—in fact, to calibrate the tube. 
To prove Boyle’s Law for pressures lower than the atmo- 
spheric pressure it is only necessary to imprison the air above the 
mercury, the closed end of the tube being then uppermost. In 
this case some mercury is first poured into the tube, and the 
floating piston then dropped in. Inverting the tube slowly, some 
air is let pass up above the mercury. The length of mercury in 
the tube is deducted from the height of the barometer for P. 
To alter this pressure a stick or glass rod is passed up the tube 
