INTRODUCTION. 



the regularity with which the air and water have followed each other 

 in temperature, leading to the belief that the former is influenced by 

 the latter in the extensive estuary which forms the Bay of Rio, the lines 

 indicating the mean daily result being almost parallel to each other. 



The sea-breeze tends very much to moderate the heat of the 

 climate. Its setting in during the time of our visit was extremely 

 irregular, sometimes not until 3 p. M., but it generally commences 

 between 9 and 10 o'clock in the morning, and its average duration is 

 from seven to eight hours; during the night a land-breeze relieves the 

 heat of the climate : the hottest part of the day is the interval between 

 these two winds. The fair days in the year are about two-thirds ; one- 

 sixth are cloudy, and a sixth rainy. During the summer months 

 little dew is said to fall, but in the winter it is reported as occasionally 

 very copious. 



EIO JANEIRO TO BIO NEGRO. The route pursued was about one 

 hundred miles from and parallel to the coast of South America. By 

 referring to the diagram on Plate IV., page 94, it will be seen that great 

 and sudden changes took place in the temperature of both air and water 

 between the latitudes of 36 and 40, which was in the immediate 

 neighbourhood of the River La Plata, and whilst crossing its mouth. 

 This temperature may mark out the width of the mouth of this river, 

 but I am inclined to consider it as the effects of a cold current from 

 the south setting in on the coast : the breadth of this space in a line 

 parallel with the coast was three hundred miles. I have been unable 

 to ascertain the temperature of the waters of the La Plata during the 

 month of January, but believe it to be higher than the point to which 

 the temperature fell. The winds during our voyage were light, from 

 the southward and eastward, and the weather generally fine. The 

 thermometer had fallen 11 in these 18 of latitude, one-half a degree 

 for each degree of latitude. The mean barometric pressure had 

 decreased -115 in. 



EIO NEQEO. Our stay at the mouth of the Rio Negro (which is the 

 northern boundary of Patagonia, lies in latitude 41 south, and is the 

 most southern of the former Spanish settlements on the eastern side 

 of South America) was too short to obtain any series of observations 

 which would increase our knoAvledge of the climate ; but as the mete- 

 orology of this part of the world is extremely interesting, and but 

 little known, I deem it advisable to give some of the results obtained 

 during the time of our detention there. 



The mean temperature of the air in the roads was 66'4 Fahr., that 



4 



