THE SMITHSONIAN INSTITUTION. 235 



fourteen mornings only. The greatest number of days in the year coin- 

 ciding in temperature at noon was sixteen, with the thermometer at 68°. 



At San Francisco, in the year 1851, there were one hundred and 

 sixty-one mornings with the temperature from 50° to 54° inclusive. 

 At Philadelphia the greatest number of mornings within the like range 

 of the thermometer, in the year 1839, was but forty-six. At the former 

 place there were two hundred and nineteen days within a range of 5° 

 at noon, while the greatest number within that range at Philadelphia 

 was but sixty. 



It is not uncommon for the thermometer at noon to stand almost at 

 the same point day after day, for one or two weeks. March, April, and 

 October, were the most irregular months in this respect, being inter- 

 spersed with a great proportion of warm days. But no other month of 

 the year elapsed without exhibiting one or two weeks continuously 

 when the mercury varied only 5°. From the 12th to the 26th Febru- 

 ary, (15 days,) the lowest mark at noonday was 55°, and the highest 

 60°. From tlie 2d to the 16th of June, (15 days,) the lowest was 65°, 

 and the highest 70°. From the 1st to the 13th of July, (13 days,) the 

 lowest was 61°, and the highest 65° ; and from the 17th to the 27th of 

 the same month, the lowest was 60°, and the highest 65*^. In the first 

 13 days of August, the lowest was 63°, and the highest 65° — a varia- 

 tion of only 2°. From the 13th to the 21st of September, the lowest 

 was 65°, and the highest 66'^ — only 1° of variation in nine days. — 

 From the 2d to the 11th of November, (10 days,) the range was be- 

 tween 62° and 67°; and from the 14th to the 24th, (11 days,) between 

 60° and 65°. From December 8th to the 24th, (15 days,) the varia- 

 tion was from 54° to 58° — only 4°. 



The sudden fluctuations of temperature incident to the climate of the 

 Atlantic States are unknown here. We have none of those angry out- 

 breaks from the northwest, which change summer to winter in a few 

 hours. But the diurnal depressions of temperature in the afternoon are 

 considerable. The average fall of the thermometer from noon to 11 p. 

 M., for the whole year, is at Philadelphia 11°; at San Francisco 14°. 

 The change at the latter place is the more striking, from its greater 

 rapidity. In the season of the sea breezes, a few hours will reduce the 

 temperature fifteen, twenty, and on some of the warmest days, twenty- 

 five degrees; and this change is effected long before sunset. Under 

 the head of winds, this subject will be more fully examined. 



Comparing one day with that succeeding, the difference is never 

 •great. The greatest difference during the year at noon, between two 

 adjoining days, was 21°. Turning to the table for 1839, at Philadel- 

 phia, in the month of March alone, three instances are found exceeding 

 this : the differences being in one case 29°, in the second 33°, and in 

 the third 35°. Though no other month was equal to March in this 

 respect, yet there were several other examples during the year which 

 exceeded the extreme at San Francisco. 



As regards the influence of the seasons on vegetation, the common 

 order is reversed. The entire absence of rain in the summer months 

 parches the soil, and reduces it almost to the barrenness of a northern 

 winter. The cold sea winds of the summer solstice defy the almost 

 vertical sun, and call for flannels and overcoats. When the winds 



