WE some months ago printed a paper describing briefly the leading features of Mr. Crookes's discovery of the mechanical action of light. We this month publish a more elaborate article under the same title, with new illustrations, in which the distinguished discoverer goes more fully into the subject, states how he was led into the investigation, explains the construction of the instrument, traces out the action of different kinds of rays, shows the value of the contrivance as a photometer or light-measurer, explains its magnetic and electrical relations and how its motions may be recorded, suggests its meteorological uses, and finally considers its results as determining the amount of the force of sunlight upon the earth. Nothing could better illustrate the wide and complex interactions 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 "Geissler's 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 motion is quickened, and when the instrument 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 aspect 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 attracting much attention from scientific men, who may be expected in due time to report the results of their own reflections and experiments upon the subject.
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 religious aspect as a tribute to the dignity, inspiration, and sacredness of conscientious and successful labor. The warriors, politicians, orators, have their honors elsewhere; the Centennial Exposition is an ovation to the "captains of industry." The multitudinous display is all due to the achievements of labor, to head-toilers and hand-toilers—the devotees and the heroes of science