INTRODUCTION. xx jjj 



Andes. The prevailing winds pass for a long distance over the sea, 

 and serve to modify the heat of a tropical climate. 



The small diagram (on Plate VIII., page 188) of the mean daily 

 temperatures at Callao, will show their variations from each other 

 and from that of the water of the bay. 



CALLAO TO TAHITI. This route was sailed over during the 

 months of July and August, and between the parallels of 13 and 18 

 south latitude. The temperature of the air and water changed and 

 the weather became fair when we had reached a distance of one 

 hundred and twenty miles from the coast; the former from 63 to 

 76, and the latter from 66 to 78, until we entered the Paumotu 

 Group. We carried the southeast trades until we were in latitude 

 17, and had reached the longitude of 110 west, when they left us, 

 and the wind changed to the west, southwest, and northwest. This 

 interruption continued for several days, with an atmosphere loaded 

 with vapours, an overcast sky, and the barometer standing '100 higher : 

 the diagram (Plate VIII., page 188) will exhibit these variations. 

 The wind afterwards could not be termed the trade wind: though 

 mostly from the eastward, it frequently veered to the southwest, from 

 which quarter the upper stratum of clouds was passing rapidly : these 

 continued until our arrival off Clermont de Tonnerre, the most eastern 

 island of the Paumotu Group. From our observations, it appears that 

 the limits of the trade winds towards the central part of the Pacific, do 

 not extend the same distance from the equator that they do nearer 

 the coasts. It will be recollected that but a month previous to our 

 passing over this route, we had met with the southeast trades in the 

 latitude of 28 30' south, near the coast of South America ; now, at a 

 distance of two thousand miles from the coast, we had lost them in 

 latitude 18 south, and at the season of the year when they are sup- 

 posed to extend to their farthest southern limit, showing a difference 

 in the distance from the equator of 10 in latitude ; and it will be 

 hereafter seen that beyond the Society Islands it is contracted within 

 still narrower limits, while on the western side of this ocean they have 

 been found to extend to the latitude of 22 south : this gives their line 

 of limit a convex curve towards the equator. The degree of mois- 

 ture which we experienced varied from '701 to '984 ; mean '873. 

 Solar radiation, sun 95, air 76'8, difference 18 % 2; greatest diffe- 

 rence 41, least 4. The water throughout was about 4 warmer 

 than the air. 



