THE SMITHSONIAN INSTITUTION. 243 



Inland, they do not extend beyond the barrier of" hills which skirt the 

 coast. A distance of fifty miles in any direction from San Francisco 

 brings you into a different climate. In a southeast course, towards 

 San Jose, you escape the winds and fogs of summer by travelling 

 twenty or tnirty miles. Even in Contra Costa, directly across the bay> 

 they are less severe, though the trees show, by their semi-prostrate 

 attitudes, the direction of the prevailing atmospheric currents. 



The general principles on which depend the diurnal currents of air, 

 which set in from sea to land, are well known. The land being more 

 heated than the ocean by the sun's rays, the superincumbent heated 

 air r ses in a steady column. Its place must be supplied from some 

 quarter, and the colder and denser air of the ocean accordingly flows 

 in, constituting a sea breeze. Independently of this, we have the 

 universal westerly current, coinciding in its course and tending to add 

 strength and constancy to the sea breeze, while the topographical fea- 

 tures of the Bay of San Francisco, and the region of country bordering 

 on it, enhance the effect. These several causes combined will explain 

 the extraordinary constancy and force of the westerly winds at this 

 point. 



The importance of these winds, in connexion with the climate of 

 San Francisco, has led me to dilate much more than I intended in 

 taking up the subject. There are other incidents of the climate 3^et to 

 be considered. 



No. 4.— RAIN, STORMS, CLOUDS, AND MISTS. 



Mining and agriculture, the leading interests of California, are inti- 

 mately connected with the distribution of rains. The absence of rain 

 during one portion of the year, and its profuse supply during the 

 remaining period, a phenomenon which maintains through a great 

 extent of the western coast of the American continent, gives the subject 

 additional interest as connected with meteorological science. These 

 considerations induce me to enter into some details which may prove 

 beneficial, although they may possibly not be so interesting to every 

 class of readers. The prevailing idea of mists and fogs, and the rela- 

 tion of the climate to health, furnish additional reasons for some degree 

 of minuteness in the investigation. 



The subjoined table presents the following details, for each month, 

 from December, 1850, to March, 1852, inclusive of both months. 



1st. column. The number of days on which rain fell. 



2d. The quantity of rain in inches. 



3d and 4th. The proportion of clear and cloudy weather. 



5th. The number of days clear, or nearly so, from sunrise to sunset. 



6th. The number of days entirely cloudy, from morning to night. 



7th, 8th, and 9th. The number of days misty in the forenoon, in the 

 afternoon, and in tlie evening. 



