INTRODUCTION. 



eastern section. The extraordinary daily rise of temperature in the 

 middle section causes great rarefaction, and in consequence a rush of 

 the aerial currents towards it. At Wallawalla, during the summer 

 months, it blows a gale of wind almost every night, but the mornings 

 are calm and pleasant. Easterly winds are seldom felt in any part of 

 western America. 



SAN FRANCISCO, CALIFORNIA. Our stay at San Francisco was 

 during a part of August, and the months of September and October. 

 The diagram on Plate XIX., page 612, exhibits the mean daily re- 

 sults. The position of the Observatory was at Sausalito, on the 

 northwest side of the Bay of San Francisco; the Table Mountain 

 (2000 feet high), rose on the west and on the east the bay extended 

 several miles; we were thus, as it were, sheltered from the prevailing 

 winds. The mean standing of the barometer was 29-896 inches, its 

 greatest oscillation being -416 in. The mean thermometer 61-37, its 

 maximum 91, minimum 47 ; in the sun it rose to 112, at the same 

 time in shade 86 ; the mean solar radiation was 9 difference with 

 shade ; the degree of moisture '701. The winds at San Francisco 

 were constantly from the northwest to southwest, and for several 

 hours daily blew a strong gale. Rain occurs only in the winter months 

 from November till March ; but frequently none falls even then, and 

 droughts sometimes prevail for upwards of a year. Cultivation is 

 entirely dependent upon irrigation. With the observations at San 

 Francisco I have brought those made at Washington City during the 

 same period, into comparison ; they are shown on Plate XVI., page 

 516. These two places are more nearly on a parallel than Cambridge 

 and Vancouver, and the positions are reversed, San Francisco being 

 situated on the seaboard in the western, while Washington lies as 

 much in the interior on the eastern as Vancouver is on the western. 

 The mean daily results, when compared, are extremely interesting. In 

 this case the barometer, as before, stands lowest on the sea-coast : the 

 mean of that at Washington being 30-058 inches, while that at San 

 Francisco is but 29-896 inches. The oscillations are also dissimilar; 

 there is scarcely a day in which they are found to agree : the tempe- 

 ratures are also dissimilar, except a few days about the autumnal 

 equinox. The mean temperature for the period at Washington was 

 61-1; maximum, 88'5; minimum, 33. The difference between San 

 Francisco and Washington is 27 for the mean; 3 for the maximum, 

 and 14 for the minimum higher at the former. The highest tempe- 

 rature at Washington was on the 30th August, while that at San 



