xxii INTRODUCTION. 



arrived at Callao ; we remained there until the middle of July, part 

 of which time our observations were made at the Island of San 

 Lorenzo, which forms the western side of the bay. This was the 

 season of winter, the sun being to the north of the line, and the time 

 of the year when the heavy mist (garua) prevails, which almost 

 screens the heavens from view. This is variously accounted for. 

 Throughout the year the wind blows almost constantly from the 

 southward, although at diiferent seasons it varies very much as to 

 strength : during the spring and summer months, from November till 

 May, it blows moderately without intermission, whilst in the autumn 

 and winter it is, though generally stronger, often interrupted, when 

 light airs succeed from the north ; on their occurrence the " garua" is 

 immediately formed, and towards evening a heavy deposition takes 

 place, which leads to the conclusion, that the returning warm moist 

 current from the north encountering the cooler ones from the south 

 causes precipitation. The recurrence of these circumstances always 

 produces the same effect. Whilst the south winds prevail conti- 

 nuously, the whole coast of Peru enjoys a clear atmosphere and fine 

 weather. In winter the sun has seldom power enough until near mid- 

 day to dissipate these mists, which renders the climate during the 

 winter season damp and cold, prejudicial alike to comfort and health. 

 The mean annual temperature at Lima is placed by the best autho- 

 rities at from 60 to 85 Fahr. I am not aware that any recorded 

 observations of the barometer have been made throughout the year, 

 but the mean standing of the mercurial column during our stay was 

 29 '9 83, and its greatest oscillation '200 : the maximum and minimum 

 temperature was 56 to 82 Fahr. ; mean temperature during our stay, 

 63-3 ; mean water 59'8, minimum 56, maximum 63 ; solar radia- 

 tion 83-1 sun, air 73-6, difference 9'7; 35 observations. Photo- 

 meter 93 and 19 ; 8 observations: the degree of moisture -922. 



From the observations made during our visit, we found the vapour 

 plain on this coast to be elevated above twelve hundred and twenty feet. 

 Below this point vegetation is confined to the winter months, unless 

 moisture is supplied by artificial means. Without the tropics, where 

 the vapour plain exists, it protects vegetation below it, but within, the 

 reverse seems to take place : in the first case, we ascend from luxu- 

 riant vegetation to barrenness; in the latter, from an arid, barren 

 waste to some of the most productive and fertile portions of the globe. 

 The low temperature of this part of Peru is to be ascribed more to the 

 influence of the temperature of the ocean, than its proximity to the 



