APPENDIX. 



447 



upon both mercurial surfaces, at a and c, but it presses upon them 

 equally, and therefore does not change the common level. Into the 

 arm A, push an air-tight piston, p, which has a valve 

 opening upward but not downward. As this piston 

 is pushed downward, the air in A escapes through 

 this valve, and p finally rests upon the surface of the A 

 mercury at a. When the piston p is subsequently 

 lifted to A, the atmospheric pressure is wholly re- 

 moved from the surface of the mercury in that arm 

 of the tube, while it acts with unchanged intensity 

 upon the surface at c. The consequence is that the 

 mercury follows the piston until there is a difference 

 of about 760 mm. or 30 inches between the levels of 

 the mercury in the two arms of the tube. If the 

 tube have a sectional area of one square inch, the 

 mercury thus supported would weigh about 15 

 pounds, and would exactly equal the weight of an 

 air column of the same sectional area, reaching from 

 the apparatus to the upper surface of the atmos- 

 phere. 



APPENDIX I. 



Magnetic Needles. Magnets may be made 

 for the experiments described in 306 by magnet- FIG. 346. 

 izing three stout knitting-needles (see 320). They 

 may be suspended by means of a triangular piece of stiff writing- 

 paper. Pass the needle through the paper near the lower corners ; 

 at the other corner affix by wax the end of a horse-hair, which will 

 exert no torsion. The poles may be indicated by little bits of red 

 and of white paper, fastened by means of wax to the ends of the 

 needles. 



APPENDIX J. 



Dipping-Needle. A dipping-needle may easily be made by 

 thrusting a knitting-needle through a cork so that the cork shall be 

 at the middle of the needle. Thrust through the cork, at right 

 angles to the knitting-needle, half a knitting-needle, or a sewing- 

 needle, for an axis. Support the ends of the axis upon the edges 

 of two glass goblets or other convenient objects (see Fig. 131). 

 Push the knitting-needle through the cork so that it will balance 

 upon the axis like a scale-beam. Magnetize the knitting-needle 

 and notice the dip. (See 314 [a].) 



