by the combined volume, mass, and rotation, and other causes,--such as the axial inclination of such planets, for example,--renders these variations in the constitution of planetary atmospheres a certainty. As Mars has a diameter much more than one-half that of the earth, and a diurnal rotational period nearly the same, while his mass, which controls the action of gravity, is only about one-ninth that of the earth (see Appleton's Cyclopædia), it is obvious that his oxygen-gathering power, compared with that for accumulating nitrogen and aqueous vapor, is much higher than that of the earth, and we should expect to find there an attenuated atmosphere very rich in oxygen, and with a relatively smaller proportion of aqueous vapor, or even water, on his surface. Such seem to be the facts as far as observed. In operating an electric machine the strength of the current is directly proportionate to the speed of rotation,--that is to say, to the velocity of the generating surface; for example, of the Wimshurst induction machine it is stated (page 63, "Electricity in the Service of Man"), "These four-and-one-half inch discharges take place in regular succession at every two and a half turns of the handle." It is also a well-established law of electrolysis that "The amount of decomposition effected by the current is in proportion to the current strength." Professor Ferguson ("Electricity," page 225) says of the voltameter, an instrument devised by Faraday, and used for testing the strength of currents by the proportionate decomposition of acidulated water, "Mixed gases rise into the tube, and the quantity of gas given off in a given time measures the strength of the current." Roughly estimating the diameter of Mars at five-eighths, the surface velocity at three-fifths, and the mass at one-ninth those of the earth, this planet should have an atmosphere containing about sixty per cent. of oxygen and forty of nitrogen, with a barometric pressure at sea-level of about six and one-half inches of mercury. This would be an excellent atmosphere,--about equal in its quota of oxygen for each respiration to that of the higher areas of Persia, a great country for roses. The aqueous vapors lying low and near the surface would serve as a vaporous screen to concentrate and retain the sun's heat and retard radiation from that planet. Nothing in particular seems to be the matter with Mars. On the contrary, the mass of Jupiter is so great, and his attraction of gravity so powerful, that it is only by his exceedingly rapid diurnal rotation (once in less than ten hours) that it is possible for him to accumulate any effective percentage of oxygen at all. But there is certainly plenty of water there.