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



shortly before 9 p.m. But it is only on the open sea, at a distance from land, where 

 this typical curve of the diurnal humidity occurs with its sino-le minimum and 

 maximum. Over land the humidity daily curve shows two well-marked minima and 

 maxima — the two minima occurring in the early morning and in the afternoon ; and the 

 more inland the situation and stronger ' the sun, the more strongly marked is the 

 afternoon minimum. Now the hygrometric observations made near land show a daily 

 humidity curve intermediate between these two. The observations made near 

 land in the North Atlantic, during the same months, disclose the following diurnal 

 variations (Plate I. fig. 4) : — 



Inch. Inch. Inch. 



2 a.m. -0-003 10 a.m. +0-014 6 p.m. 0-000 



4 „ -0-009 Noon +0-010 8 „ -0-004 



6 „ -0010 2 p.m. +0-007 10 „ -0-005 



8 „ -0-003 4 „ +0-015 Midt. -0-007 



The disturbance due to proximity to land in the diurnal distribution of the aqueous 

 vapour in the lower stratum of the atmosphere is remarkable. The maximum and 

 minimum no longer follow the corresponding phases of the temperature of the surface 

 of the sea and the air. The disturbing agents are the land and sea breezes, with the 

 other atmospheric movements resulting from the unequal heating of land and water. 

 Under the influence of the land breeze, the time of minimum humidity is delayed till 

 about 6 a.m. The most remarkable feature of the curve, however, is the occurrence of 

 a secondary minimum of humidity, for some hours between 10 a.m. and 4 p.m., a feature 

 altogether absent in the atmosphere over the open sea. It is to be noted that this 

 mid-day minimum occurs at the hours of the day when, the surface of the land being 

 most highly heated, the ascending current of heated air rising from it is strongest, and 

 the resulting breeze from the sea towards the land therefore also strongest. This 

 diminution in the amount of aqueous vapour, recorded on board the Challenger when 

 near land, points unmistakably to an intermixture, with the air forming the sea 

 breeze, of descending thin air filaments or currents to take the place of the masses of 

 air removed by the currents which ascend from the heated surface of the land ; and 

 this increased dryness occurs also in the air of the sea breezes as they near the land. 



The relative humidity of the air, or, as it is more frequently called, its humidity, is 

 the degree of its approach to complete saturation. Complete saturation is represented 

 by 100, and air absolutely free of vapour by 0. The latter, however, never occurs in 

 the free atmosphere. About the lowest relative humidity, or driest state of the 

 atmosphere hitherto recorded with the requisite care and accuracy, was 6 per cent, at 

 the Ben Nevis Observatory at 8 p.m. of March 12, 1886, on which day the mean of the 

 twenty-four hourly observations was only 15 per cent. 



The diurnal variation of the relative humidity, which is quite different from that 



