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



tions. These observations are by far the most complete yet made on the meteorology 

 of the ocean. 



In discussing the observations, the first step was to recopy the whole of them, 

 arranged chronologically as observed, according to the hours of observation, a distinct 

 set thus arranged being set apart for each of the subjects observed, viz. the barometer, 

 dry bulb, wet bulb, maximum and minimum thermometers, wind, cloud, weather, 

 scmalls, thunder and lightning, and the temperature of the surface of the sea. The 

 hourly values were then calculated for periods of time varying from three days to a 

 month, and from these results hourly values for still longer periods were calculated, 

 with the view of showing the distribution of the phenomena, as influenced by proximity 

 to, or distance from, land, by latitude, by season, and in some cases by the extent of 

 water surface presented by the five great oceans of the globe. 



Diurnal Phenomena. 



Variation of Temperature. — Of the daily changes which take place in the 

 atmosphere, the first place must be assigned to those which relate to temperature, 

 since on these changes all others are either directly or indirectly dependent. 

 Observations of the temperature of the air, and of the water and land surfaces of the 

 earth, are thus of fundamental importance in meteorology. 



The sun's rays which fall on land and on solid bodies generally are wholly 

 absorbed by the thin surface layer exposed to the heating rays, the temperature of 

 which consequently rises. While the temperature of the surface thus increases, a wave 

 of heat is propagated downwards through the soil. The intensity of this daily wave of 

 temperature rapidly diminishes with the depth, so that it ceases to be measurable about 

 four feet below the surface. Part of the heat thus imparted to the surface layers is 

 conveyed away upwards by convection currents, which originate in the superheating of 

 the lowermost stratum of air in direct contact with the heated surface of the land. At 

 the same time colder currents descend from greater heights to fill the space left by the 

 ascending currents. 



Quite different, however, is the influence of the sun's rays on water. In this case 

 the solar rays are very far from being altogether arrested at the surface, but penetrate 

 to a very considerable depth. The heating effect of the sun has been shown by the 

 observations of the Challenger to be distinctly appreciable to a depth of 500 feet 

 below the surface of the sea. 



The rate at which, in clear water, the heating effect takes place at different depths 

 is a problem not yet worked out, being very difficult of even an approximate solution 

 owing to the presence of the relatively large numbers of dust particles which even the 

 clearest water contains. Since water is a bad conductor, the heat thus distributed 



