154 National Geographic Magazine. 



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 35° 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°. 



Giro 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-heating 

 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, 1S74 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. 



