446 APPENDIX. 



base- File the other end of the wire to a sharp point. For a few 

 cents, such a wire with an iron base may be had ready made at the 

 stationer's. Pass the straight tube of the apparatus over this wire 

 until the closed tube rests upon the sharpened point. The chimney 

 with its four horizontal arms is now delicately suspended, free to 

 revolve in stable equilibrium. Place the apparatus in the middle of 

 a tub and pour water into the open end of the chimney. Tour 

 wheel will work as well as Edgerton's or Ritchie's. The satisfac- 

 tion of seeing the machine work and knowing that you made it will 

 amply repay the cost, leaving the instruction and added skill for 

 clear profit. 



APPENDIX G. 



Balloons. (See 272.) A little thought concerning the full 

 meaning of Archimedes' Principle will show that if a body weighs 

 less than its own bulk of air it will rise in the air. Thus soap- 

 bubbles filled with hydrogen or other light gas will ascend. If the 

 bubble be made from hot water and filled with warm air it will 

 rise ; if it be made from cold water and filled with cold air it will 

 fall. (Explain why.) The same principle applies to balloons. A 

 balloon will support a weight equal to the difference between the weight 

 of the balloon with the contained gas and the weight of the air dis- 

 placed. A liter of hydrogen weighs 0.0896 g. ; a liter of coal gas, 

 from 0.45 g. to 0.85 g. ; a liter of air heated to 200 Centigrade, about 

 0.8 g. During the siege of Paris in 1870, the Parisians communi- 

 cated with the outer world by means of balloons about 50 feet in 

 diameter, having a capacity of about 70,600 cu. ft. These balloons 

 with net and car weighed about 1000 pounds each and had a carry- 

 ing ability of about 2000 pounds. Balloons have been made about 

 100 feet in diameter, having a capacity of about half a million cubic 

 feet. In 1861, an ascent was made to a height of seven miles. 



APPENDIX H. 



Atmospheric Pressure. (See 275.) Into a bent glass 

 tube, ACS, pour mercury to a height of about 20 inches, or 50 cm. 

 The mercury will, of course, stand at exactly the same level, ac, in 

 the two branches. If equal pressures of any kind be exerted upon 

 the surfaces of the mercury at a and c, this level will not be dis- 

 turbed, while any difference of pressure would be promptly shown 

 by the movement of the mercury and a consequent difference in. 

 the heights of the two mercury columns. The atmosphere pi 



