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



period differed considerably from that at the close, a correction has been applied so as 

 to give the true deviations from the daily means for the observed hours. It will be 

 seen, from an examination of the Table, that in nearly all cases the short periods 

 adopted give results closely approximate to the mean diurnal variations. In a very 

 few cases, such as the period of six days from October 3 to 8, 1875, when the 

 Challenger was in the South Pacific, mean position lat. 21° 20' S., lono-. 149° 40' W. 

 the usual distribution of the daily temperature was reversed, the maximum occurrino- 

 early in the morning, and the minimum early in the afternoon. All such anomalous 

 results were due to the track of the Challenger at the time being across a succession 

 of warm and cold currents, the differences of whose temperatures, combined with the 

 hours of the day at which the observations were made, gave the curve of the diurnal 

 variation for the period its anomalous character. 



The diurnal variation of the temperature of the surface of the North Atlantic has 

 been calculated from the observations made on the one hundred and twenty-six days 

 from March to August 1873, and in April and May 1876, the mean latitude of all the 

 points of observation being nearly 30° N. and the longitude 42°. The following are 

 the two-hourly deviations from the mean (Plate I. fig. 1 ) : — 



2 a.m. -0°-2 10 a.m. 0°-l 6 p.m. 0°-3 



4 „ -(T-3 Noon 0°-2 8 „ (TO 



6 „ -0°-3 2 p.m. 0°-5 10 „ -0°-2 



8 „ -0°-l 4 „ 0°-5 Midt, -0°-3 



Thus in Mid Atlantic, about lat. 30° N, where the sun's heat is strong, and at the 

 time of the year when the sun is north of the equator, the diurnal fluctuation of the 

 temperature of the surface is only o> 8. Similarly in the South Atlantic, lat. 33° S. and 

 long. 20° W., the mean diurnal fluctuation is 0°"8, or the same as in the North 

 Atlantic. In the North Pacific, near lat. 37° N. and long. 170° W. 5 it is l°-0 ; and in 

 the South Pacific, near lat. 36° S. and long. 87° W., it is 0°"9. Hence the general 

 diurnal range of temperature near the centres of these four great oceans, and near the 

 summer solstice, is a little less than a degree. On the other hand, near the equator 

 both in the Atlantic and Pacific the diurnal range is only o, 7, being thus 0°-2 less 

 than about lat. 36°, a difference probably due to the more clouded skies and less 

 sunshine of equatorial regions. In February 1874, when the mean position of the 

 Challenger was nearly lat. 61° S., the difference between the mean coldest and warmest 

 hour was only 0°'2. The mean daily range deduced from the whole of the observations 

 made during the three years and a half is o> 8. 



The small daily variation of the temperature of the surface of the sea shown by 

 the Challenger observations is unquestionably a most important contribution to 

 physical science, forming in truth one of the prime factors in meteorology, particularly, 

 as will appear further on, in the discussions relating to atmospheric pressure and winds. 



