Scientific conditions have so changed that in these later years it becomes more and more difficult for investigators to publish any work which may be characterized as magnum opus. Under this head, however, must be classed Buchan's important memoir on the distribution of atmospheric pressure, temperature, and wind direction over the whole world; a large quarto volume, which contains much new material. It has been incorporated with the results of observations during the Challenger expedition, in which series this work appears. The isobars and isotherms for each month in the year for the whole earth are charted on Mercator's projection, and for the northern hemisphere on a chart constructed on a polar projection. In connection with an abstruse subject, to which Buchan has paid so much attention, the diurnal variation of pressure, he opines from the Challenger observations that the oscillations are due to the heat taken from the solar rays directly in passing through the air and instantaneously communicated through the whole mass from top to bottom by heating and evaporation of water on innumerable dust particles. The afternoon minimum, he thinks, is caused by upward currents removing a portion of the lower air. Marked differences exist between the continental and insular types, since on islands the morning minimum is unusually large and the afternoon minimum so small as to disappear, while in continental types the reverse conditions obtain.
Werner Von Siemens, in answering Sprung's criticism on his general air currents, after repelling certain statements of Sprung, describes his own theories, which are worthy of restating:
1. All winds are caused by the disturbances of indifferent equilibrium, and the motion of the air is to restore equilibrium.
2. These disturbances are caused through overheating of the layers of air near the surface of the earth by insolation, through unsymmetrical cooling of the higher layers by radiation, and through the heaping up of air masses caused by obstructions.
3. The disturbances are adjusted by ascending currents, wherein the particular species of acceleration occurs in which the increase of velocity is proportioned to the diminution of pressure.
4. The upward currents correspond to equally great descending currents in which there is a decrease of velocity corresponding to the acceleration in the upward velocity.
5. If the region of overheating of the air is limited locally, a local upward current reaching to the highest layers of air arises, and whirlwinds appear with interior spirally ascending currents and outside similar spiral descending currents. The result of this is dispersion of the superfluous heat of the lower air by which the adiabatic equilibrium is disturbed throughout the whole column of air taking part in the whirling motion.
6. In case the region of disturbance of the indifferent (or adiabatic) equilibrium is very extensive, as, for example, the whole of the tropical zone, the temperature adjustment can no longer be accomplished by locally ascending whirls, and a whirling current must then arise involving the whole atmosphere. The same conditions apply to these as to the local whirls of accelerated upward motion and retarded descent in such a manner that the velocity at different altitudes arising from heat converted to work is approximately proportional to the prevailing pressure at the place.
7. In consequence of the meridional motion produced and maintained by conversion of heat into work, the whole atmosphere in every latitude must rotate with approximately the same absolute velocity. Thus the meridional currents produced by overheating combine with the currents embracing the whole wind system of the earth, with the result of disseminating the excess of temperature and humidity of the torrid zone over the temperate and arctic zones, thereby producing the prevailing winds.
8. This is accomplished by the production of alternating local depressions and elevations of barometric pressure by the disturbance of indifferent equilibrium in the upper layers of the air.