20 THE VOYAGE OF H.M.S. CHALLENGER. 



and the evening maximum by accessions to the mass of atmosphere overhead from this 

 upper current. 



The Challenger observations all show that over the ocean, latitude for latitude, 

 the amplitude of the oscillations is larger in an atmosphere highly charged with aqueous 

 vapour, and less in a dry atmosphere. Also over the open sea, the morning minimum 

 is largest in equatorial regions, and it diminishes with latitude ; but its rate of 

 diminution with latitude through anticyclonic and other regions is generally less and 

 more uniform than is the case with the afternoon minimum. 



From October 12 to 22, 1875, the mean pressure in lat. 35° 1' S. and long. 

 134° 35' W. was 30 - 298 inches, and the difference between the mornino- maximum 

 and the afternoon minimum was only - 036 inch ; again, from July 12 to 19 in the 

 same year, in lat. 36° 16' N. and long. 156° 11' W., the mean pressure was 30"328 

 inches, and the difference between the a.m. maximum and the p.m. minimum was only 

 0025 inch. Thus in the Pacific about lat. 35°-36° N. and S., with a mean pressure 

 much greater than near the equator, the oscillation is much less, being in the North 

 Pacific less than a third of what occurs near the equator. In the same latitudes in 

 the middle of the South Atlantic the difference was observed to be 0"025 inch, and 

 in the North Atlantic 0*014 inch. Now these are regions of the four great oceans 

 which are overspread by permanent anticyclones, and characterised by calms, light 

 and variable winds, and the central regions of which are as a matter of fact but little 

 traversed by sailors, as is well shown on Baillie's Meteorological Charts of the Oceans. 

 These regions are shown on the Isobaric maps, and it will be seen that the surface winds 

 outflow in every direction from the high pressure areas of the anticyclones. Since, 

 notwithstanding the outflow of the surface, pressure remains high, it necessarily follows 

 that the high pressure is kept up by an inflow of upper currents. As the slow descending 

 air of the centre of the anticyclones connects the inflowing upper currents with the 

 outflowing winds of the surface, it follows that the air filling the central areas of the 

 anticyclones is relatively very dry, — every stage of its descent adding to its relative 

 dryness, — and contains in all probability fewer dust particles than elsewhere. Hence 

 over anticyclonic areas the atmosphere will be less cooled by nocturnal radiation 

 and less heated by solar radiation, and the change of the aqueous vapour from the 

 gaseous to the liquid state and vice versa will be also less than elsewhere. It follows that 

 the amplitudes of the oscillation will diminish as the ocean becomes more land-locked 

 with continents, in other words, as the anticyclonic region becomes better defined and 

 currents of air, which rise from the heated surfaces of the adjoining continents, are 

 poured down more steadily and copiously by the upper currents of the atmosphere. 

 Hence of the four oceans, the smallest oscillation, 0"014 inch, is shown in the 

 anticyclonic region of the North Atlantic, and the largest, 0-036 inch, in that of the 

 South Pacific. 



