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



But as regards the winds recorded by the Challenger when near land, the 

 velocity at the different hours of the day gives a curve, for the five oceans combined, as 

 clearly and decidedly marked as the diurnal curve of temperature (Plate II. fig. 23). 

 The minimum occurs from 2 to 4 a.m., and the maximum from noon to 4 p.m., the 

 absolute highest being at 2 p.m. The curves for each of the five oceans give one and 

 the same result, viz., a curve closely accordant with the diurnal curve of temperature. 

 The differences between the hour of least and that of greatest velocity are, for the 

 Southern Ocean, 6j miles ; South Pacific, 4i miles ; South Atlantic, 34; miles ; North 

 Atlantic and North Pacific, 3 miles per hour. 



In the case of each ocean, the velocity of the wind over the open sea is 

 considerably in excess of that near land, and it is noteworthy that in no case does 

 the maximum velocity near land, attained near noon, reach the velocity over the open 

 sea. The nearest approach, at any hour of the day, of the maximum velocity near land 

 to the velocity over the open sea at the same hour is in the North Atlantic, 2*5 ; South 

 Atlantic, 3 - 8 ; North and South Pacific, each 4"6 ; Southern Ocean, 5'1 ; and the mean 

 of all the oceans, 5 "6. The difference is greatest at 4 a.m., when it is about 6 miles 

 an hour, but diminishes as temperature rises, till at 2 p.m. it is less than 3 miles 

 an hour. 



On land the diurnal variation in the wind's velocity becomes more pronounced. 

 At Batavia the minimum occurs in all months in the early morning, when the tempera- 

 ture is lowest, and the maximum from 1 to 3 p.m., the minimum and maximum 

 velocities being to each other as 1 to 21. At Mauritius the minimum, occurring from 

 2 to 3 a.m., is nearly 9 - 7 miles an hour, and the maximum 18*5 miles from 1 to 2 p.m. 

 At Coimbra the maximum is five times greater than the minimum velocity in summer, 

 but in winter it is only about a half more. At Valentia, in the south - west 

 of Ireland, the minimum is 10 miles an hour at 11 p.m., and the maximum 18 miles 

 at 1 p.m. 



From a discussion of a number of places in northern Europe, Hann has shown that 

 the velocity is doubled from the minimum with a completely clear sky, three-fourths 

 greater with a sky half-covered, but with a sky wholly covered it is only one-half more. 

 At the strictly continental situation of Vienna, with a clear sky the velocity is doubled, 

 with a sky half-covered it is two-thirds greater, but when the sky is quite covered the 

 variation in the wind's velocity becomes irregular and faintly marked. This last 

 result, and the fact that the time of maximum velocity is not coincident with that 

 of the highest air-temperature, but shortly after midday, when insolation is strongest, 

 and the fact of no variation occurring over the open sea, point to the conclusion that 

 the diurnal variation is a consequence of the diurnal variations which take place in the 

 temperature of the earth's surface over which the winds blow. 



There is another class of observations which form a valuable contribution to this 



