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THE POPULAR SCIENCE MONTHLY. 



EDITOR'S TABLE. 



TEE RADIOMETER. 



TTTE some months ago printed a 

 VV paper describing briefly the 

 leading features of Mr. Orookes's dis- 

 covery of the mechanical action of light. 

 We this month publish a more elabo- 

 rate article under the same title, with 

 new illustrations, in which the distin- 

 guished discoverer goes more fully into 

 the subject, states how he was led into 

 the investigation, explains the construc- 

 tion of the instrument, traces out the 

 action of different kinds of rays, shows 

 the value of the contrivance as a pho- 

 tometer or light-measurer, explains its 

 magnetic and electrical relations and 

 how its motions may be recorded, sug- 

 gests its meteorological uses, and final- 

 ly considers its results as determining 

 the amount of the force of sunlight 

 upon the earth. Nothing could better 

 illustrate the wide and complex inter- 

 actions and dependencies of natural 

 phenomena than the circle of questions 

 that is opened by the introduction of 

 this ingenious invention. 



The radiometer (so called because 

 of its capacity of measuring radiations) 

 is a very simple instrument (as will be 

 seen by referring to Fig. 8, page 269), 

 consisting of a small glass globe from 

 which the air has been pumped out, 

 and containing four arms supported at 

 the centre by a fine point, and carrying 

 at their extremities thin vanes or disks, 

 white upon one surface and dark upon 

 the other. When light from any source 

 falls upon it the arms begin to revolve, 

 the white surfaces approaching the 

 light and the dark surfaces receding 

 from it as if repelled or pushed away. 

 We have before us a radiometer made 

 by Geissler, the inventor of " Geisslers 

 Tubes," which consists of a globe two 

 and three-quarters inches in diameter, 

 with its downward stem resting on a 



wooden base, the whole being ten inches 

 high. It is in motion constantly in the 

 daytime, propelled by the diffused light 

 from the window, and, if the curtain 

 be dropped and the room darkened, the 

 faint light that comes in at the side 

 maintains it in slow revolution. As the 

 intensity of the light increases the mo- 

 tion is quickened, and when the instru- 

 ment is placed directly in the solar 

 rays the revolutions are so rapid that 

 they cannot be counted. Mr. Crookes 

 made one instrument so delicate that 

 a single candle would drive it at forty 

 revolutions per second. 



In the hands of many the radiometer 

 is now only a curiosity and a toy, yet 

 to the physicist it is an instrument of 

 great interest as displaying a new as- 

 pect of dynamical phenomena, and may 

 help to explain still further the nature 

 of the radiant forces, and perhaps throw 

 light upon other questions. It is at- 

 tracting much attention from scientific 

 men, who may be expected in due time 

 to report the results of their own re- 

 flections and experiments upon the sub- 

 ject. 



SUNDAY AT THE CENTENNIAL EXHI- 

 BITION 



The exhibition at Philadelphia has 

 many features of interest, one of the 

 highest of which is that it stands out 

 before the world in a moral and reli- 

 gious aspect as a tribute to the dignity, 

 inspiration, and sacredness of conscien- 

 tious and successful labor. The war- 

 riors, politicians, orators, have their 

 honors elsewhere; the Centennial Ex- 

 position is an ovation to the " captains 

 of industry." The multitudinous dis- 

 play is all due to the achievements of 

 labor, to head-toilers and hand-toilers 

 the devotees and the heroes of science 



