688 



THE POPULAR SCIENCE MONTHLY. 



heat or light falling on one of the extremities of the bar of i)ith gives 

 a movement indicating attraction. When the apparatus is exhausted 

 until the barometric gauge shows a depression of twelve millimetres 

 below the barometer, neither attraction nor repvilsion results when ra- 

 diant light or heat falls on the pith, but, when the vacuum is as good 



Fig. 3. 



as the pump will produce, strong repulsion is shown when radiation is 

 allowed to fall on one end of the index. An apparatus of this kind, 

 constructed with the proper precautions, and sealed oft' when the vac- 

 uum is perfect, is so sensitive to heat that a touch with the finger on 

 a part of the globe near one extremity of the pith will drive the index 

 round over 90, while it follows a piece of ice as a needle follows a 

 magnet. With a large bulb, very well exhausted, and containing a 

 suspended bar of pith, a somewhat striking efiect is produced when a 

 lighted candle is placed about two inches from the globe. The pith- 

 bar commences to oscillate to and fro, the swing gradually increasing 

 in amplitude until the dead-centre is passed over, when several com- 

 plete revolutions are made. The torsion of the suspending fibre now 

 ofters resistance to the revolutions, and the bar commences to turn in 

 the opposite direction. This movement is kept up with great energy 

 and regularity as long as the candle burns. 



For more accurate experiments I prefer making the apparatus dif- 



