18 THE VOYAGE OF H.M.S. CHALLENGER 



different results would follow if the temperature of ouly a section of the atmosphere 

 were suddenly raised, such as the section, resembling the " lith " or division of an orange, 

 comprised between 150° and 180° west longitude. The immediate effect would be an 

 increase of barometric pressure from the expansion due to the higher temperature, 

 and a subsequent effect would be the setting in of an ascending current, more or less 

 powerful in proportion to the differences between the temperature of the heated section 

 and that of the air on each side. These are essentially the conditions under which 

 the morning maximum and the afternoon minimum take place. 



The earth makes a complete revolution round its axis in twenty-four hours, 

 and in the same brief interval the double-crested and double-troughed atmospheric 

 diurnal tide makes a complete circuit of the globe. The whole of the diurnal 

 phenomenon of the atmospheric tides is therefore rapidly propagated over the surface 

 of the earth from east to west, and, as the movement of the surface is necessarily most 

 rapid in equatorial regions, the amplitude of the oscillations there is greater than in 

 higher latitudes under similar astronomical, geographical, and atmospherical conditions. 



The Morning Minimum. — This depression of the barometric curve occurs from a 

 little before midnight to near sunrise, or during the time when the effects of nocturnal 

 radiation in lowering the temperature are the greatest. Pressure falls to the minimum 

 about four in the morning. 



Assuming that aqueous vapour in its purely gaseous state is as diathermanous as 

 the dry air of the atmosphere, let us consider the part played by the dust particles 

 suspended in the air. As nocturnal radiation proceeds, the temperature of each dust 

 particle continues to fall below that of the air immediately surrounding it. From this 

 state of things two important consequences follow — 1st, the temperature of the whole 

 atmosphere falls, and 2nd, as soon as the temperature of the dust particle reaches, in its 

 cooling, the dew point of the air in contact with it, dew begins to be deposited on it, 

 and the vitally important result follows that a portion of the aqueous vapour of the 

 atmosphere passes from the gaseous to the liquid state, thus reducing the tension. Hence 

 the morning minimum is due to a reduction of tension brought about by a compara- 

 tively sudden lowering of the temperature of the air itself and by a change of a portion 

 of the aqueous vapour from the gaseous to the liquid state. Since this takes place at a 

 more rapid rate than is compensated for by aDy mechanical or tidal movement of the 

 atmosphere from the regions adjoining, owing to the inertia and viscosity of the air, 

 pressure continues to fall to the morning minimum, which occurs some time before 

 sunrise, or rather before dawn. It is probable that the commencement of the increase 

 from the minimum before the air is yet heated by the indirect or direct rays of the 

 returning sun is due to the setting in of a mechanical or tidal movement of the con- 

 tiguous air towards this region where the pressure has been lowered. The morning 

 minimum is thus due, not to any removal of the mass of air overhead, but to a reduc- 



