12 THE POPULAR SCIENCE MONTHLY. 



spindles above, and is connecting the pulleys by the usual belts, when 

 some stern necessity requires him to transmit all the energy with cob- 

 webs. Of course, a good fairy comes to his aid, and what does she do ? 

 Simply makes the cobwebs indefinitely strong. So the physicists, not 

 to be outdone by any fairies, make their ether indefinitely elastic, and 

 their theory lands them just here, with a medium filling all space, thou- 

 sands of times more elastic than steel, and thousands on thousands of 

 times less dense than hydrogen gas. There must be a fallacy somewhere, 

 and I strongly suspect it is to be found in our ordinary materialistic 

 notions of causation, which involve the old metaphysical dogma, "nulla 

 actio in distans" and which in our day have culminated in the famous 

 apothegm of the German materialist, " Kein Phosphor kein Gedanke." 



But it is not nry purpose to discuss the doctrines of causation, and 

 I have dwelt on the difficulty, which this subject presents in connection 

 with the undulatory theory, solely because I wished you to appreciate 

 the great interest with which scientific men have looked for some direct 

 manifestation of the mechanical action of light. It is true that the 

 ether-waves must have dimensions similar to those of the molecules 

 discussed above, and we must expect, therefore, that they would act 

 primarily on the molecules and not on masses of matter. But still the 

 well-known principles of wave-motion have led competent physicists to 

 maintain that a more or less considerable pressure ought to be exerted 

 by the ether-waves on the surfaces against which they beat, as a par- 

 tial resultant of the molecular tremors first imparted. Already, in the 

 last century, attempts were made to discover some evidence of such 

 action, and in various experiments the sun's direct rays were concen- 

 trated on films, delicately suspended and carefully protected from all 

 other extraneous influences, but without any apparent effect ; and thus 

 the question remained until about three years ago, when the scien- 

 tific world were startled by the announcement of Mr. Crookes, of Lon- 

 don, that, on suspending a small piece of blackened alder-pith in the 

 very perfect vacuum which can now be obtained with the mercury- 

 pump, invented by Sprengel, he had seen this light body actually re- 

 pelled by the sun's rays; and they were still more startled, when, after 

 a few further experiments, he presented us with the instrument he 

 called a radiometer, in which the sun's rays do the no inconsiderable 

 work of turning a small wheel. Let us examine for a moment the con- 

 struction of this remarkable instrument. 



The moving part of the radiometer is a small horizontal wheel, to 

 the ends of whose arms are fastened vertical vanes, usually of mica j 

 and blackened on one side. A glass cap forms the hub, and by the 

 glass-blower's art the wheel is inclosed in a glass bulb, so that the cap 

 rests on the point of a cambric needle ; and the wheel is so delicately 

 balanced on this pivot that it turns with the greatest freedom. From 

 the interior of the bulb the air is now exhausted by means of the 

 Sprengel pump, until less than y^Vo" ^ the original quantity is left, 



