National Geographic : 1889 Apr
154 National GeographicMagazine. fluid entering fluid at rest, the development of discontinuous (so called by Helmholtz) currents, the tendency of parallel currents of unequal velocities towards similar velocities, the effect of fric tion arising from contiguous currents of different velocities, upon the coefficient of friction, of the temperature distribution over the surface of the earth, etc. He derives three very simple ex pressions for the motions of the air; the first giving the velocity in a vertical direction at any point, in terms of latitude, and a constant and factor depending on the distance of the point above the surface of the earth. The other expressions give the veloci ties in a north or south direction, and in an east or west direction, also in terms of constants and latitude. The velocity when charted from Overbeck's equations indicate an ascending vertical current from the equator to 350 north, and thence a descending current to the pole. The meridional current at the equator and pole are zero, and have a maximum value at latitude 45°. Ciro Ferari, from long and important investigations of thun der-storms, shows that these phenomena invariably attend motion less areas of low pressure, and believes the surest elements for predicting such storms will be found to be the peculiarities in distribution of temperature and absolute humidity. He observes that the storm front invariably tends to project itself into the re gions where the humidity is greatest, and that hail accompanies rapidly moving storms of deep barometric depression. Ferari considers the chief causes of thunder storms to lie in the connec tion of high temperature and high humidity. Grossman believes that ascending moist-laden currents are the cause of thunder storms, and hence they are most frequent when the temperature diminution with altitude is very great, so that the over-heatiig of the lower air strata in the warmest part of the day is the cause of the primary maximum of thunder-storm frequency. Abercromby and Hildebrandsson have renewed their recom mendations for a re-classification of clouds in ten fundamental types, in which the first part of the compound name, such as cirro-stratus, cirro-cumulus, etc., is to be in a measure indicative of the height of a cloud. Hildebrandsson has charted the differences of monthly means of air pressure for January, 1874 to 1884. In January, 1874, the values at nearly all the stations in the Northern Hemisphere, were plus, and those in the Southern, minus. It is to be hoped that such general discussions of this important meteorological element may be continued.