Intelligence and Miscellaneous Articles. 251 



hinder side o£ the box, or, on the contrary, in the immediate 

 vicinity of the glass ; while it attains its maximum when the pen- 

 dulum is nearly in the centre of the box, at an equal distance from 

 its two opposite sides. 



The thickness of the stratum of air between the pendulum and 

 the sides of the box exerts, therefore, a great influence on the 

 amplitude of the deflection, as it ought if this is due to the afflux 

 of the air toward the warmer region of the interior atmosphere. 

 By causing the rays to act alternately on the two sides of the pen- 

 dulum, we can moreover ascertain that the deflection of the mirror 

 certainly takes place always in. the direction in which the colder 

 air, situated behind the pendulum, goes to replace that which re- 

 ceives directly the radiations of the lamp. 



But the heat from this has not only the effect of heating the 

 air which it traverses, but the side of the pendulum which receives 

 the rays is also not slow to be itself affected by its influence. Its 

 temperature soon rises above that of the surrounding air ; and a 

 second set of currents are produced, going from the glass to the 

 mirror and tending to turn it in the opposite direction to the former 

 ones, which are directed from the posterior side of the box toward 

 the glass plate. 



The result is that the deflection due to the first currents dimi- 

 nishes, and finally changes its direction : this always takes place 

 when the rays from the lamp fall directly on one of the sides of the 

 pendulum to the right or the left of the mirror. But it is easy to 

 prevent this diminution and inversion by concentrating the lumi- 

 nous rays by a lens on one of the sides of the pendulum, while the 

 lamp heats directly the air situated on the other side. 



In this case the currents of the second set evidently tend to 

 turn the pendulum in the same direction as those produced by the 

 direct heating of the air; and there ensues an increase instead of a 

 diminution of the deflection. 



The amplitude of the deflection of the mirror depends also no- 

 tably on the angle at which it receives the rays — which is readily 

 accounted for, since the intensity of the thermic action is itself in 

 correspondence with the direction of those rays. 



In fine, it is intelligible that the heating of the air must diminish 

 with its density, while that of the pendulum must become more 

 rapid and more intense in proportion as the air contained in the 

 box is rarefied. Moreover Dr. Neesen has established that the 

 deflection corresponding to the first set of currents diminishes in 

 proportion as vacuum is produced in the apparatus, and is at last 

 entirely supplanted by the opposite deflection resulting from the 

 heating of the pendulum by the radiation from the lamp. Thus, 

 according to him, is to be explained the fact observed by Mr. 

 Crookes, from which he inferred that the action of heat on the 

 pendulum is attractive in air, while it is repellent in vacuo. 



Certainly Dr. Keesen's experiments tend to a rational explana- 

 tion of the phenomena discovered by Mr. Crookes; nevertheless 

 they cannot be considered altogether conclusive, since they were 

 not performed in a medium reaching an extreme degree of rare- 



