234 NINTH ANNUAL REPORT OF 



43°. The difference between January and August, the coldest and 

 warmest months at San Francisco, was not quite 13° ! 



To facihtate comparison, I insert the mean temperature for a series 

 of years of the several months at Philadelphia: January, 30!^°; Febru- 

 ary, 31fo; March, 40°; April, 50°; May, 60p ; June, 69°; July, 

 73^° ; August, 71^° ; September, 64° ; October, 53° ; November, 42^° ; 

 December, 33f°. By observing that one degree of latitude makes 

 about one degree of difference in temperature in the Atlantic States, 

 the reader may easily compute, from these data, the mean temperature 

 at any given place. For example, if the place be two degrees north 

 of Philadelphia, you will find its mean temperature by deducting two 

 from the temperature at Philadelphia; if south, by adding. 



The coldest month in 1851, at San Francisco, (viz: January,) was 

 9° warmer than the average of the coldest month at Philadelphia; 

 while August, the warmest month at San Francisco, was 11° colder 

 than the average of July, the warmest month at Philadelphia. 



At San Francisco, the temperature falls more rapidly in the after- 

 noon and evening than in the Atlantic States, but less rapidly during 

 the night. From 11 p. m. to sunrise, the mercury at Philadelphia falls 

 four or five degrees on the average, while at San Francisco the differ- 

 ence was less than two degrees, and in four of the months less than one 

 degree ; except in the winter, when the change is similar in this respect 

 at the two points. In November, December, and January, at San 

 Francisco, the thermometer falls from two to three and a half degrees 

 between 11 p. m. and sunrise. The same is true precisely of Phila- 

 delphia. But while at Philadelphia in all the other months the fall 

 during the same period of the night is twice as great, it is less than 

 half as much at the former place. In other words, the temperature 

 falls in the night, alter 11 p. m., four times as much at Philadelphia as 

 at San Francisco, from February to September inclusive. 



In the summer months there is scarcely any change of temperature 

 in the night. The early morning is sometimes clear, sometimes cloudy, 

 and always calm. A few hours after sunrise the clouds break away, 

 and the sun shines forth cheerfully and delightfully. Towards noon, 

 or most frequently about 1 o'clock, the sea breeze sets in and the 

 weather is completely changed. From 60° to Qb^ the mercury drops 

 forthwith to near 50°, long before sunset, and remains almost motion- 

 less till next morning. This is the order of things in three days out of 

 four in June, July, and August. May and September exhibit something 

 of the same character, the sea winds establishing themselves in the 

 former and declining in the latter month. This subject will be more 

 fully investigated under the head of winds. 



The remarkable uniformity of temperature at San Francisco maybe 

 further illustrated by taking note of the number of days in the year 

 which give the same degree. The most frequent temperature at sun- 

 rise was 53°, the mercury standing at that point on forty-five mornings. 

 The most frequent at noon was 64°, forty-two days showing that tem- 

 perature. Referring to my journal kept at Philadelphia, I happened 

 to open at the year 1839, which exhibits a fair representation of the 

 climate there. I find the most frequent temperatures at sunrise were 

 52° and 68°, but that the mercury stood at each of these points on 



