292 METEOEOLOGY. 



exceeding that of the mean annual. Sometimes a greater degree of 

 cold is experienced in March than in February, and at other times 

 spring is as well advanced in March as at other seasons it is in May. 

 In 1855 the mean temperature of May was 3° lower than that of 

 April, 1857, notwithstanding, the distribution of heat for the three 

 months of spring is marked by no great variability in successive years, 

 nor great constant diiferences of the mouths. The measure ol' heat 

 increases very gradually from month to month. Indeed, the same 

 uniformity of temperature is found to obtain throughout all the 

 meteorological seasons. In summer the greatest vicissitudes of tem- 

 perature are found to occur, as is readily seen in the subjoined table 

 for 1856 and 1857. The commencement of autumn is quite similar 

 to the beginning of spring in its mean of daily temperature. The 

 earth remaining warmer than the atmosphere under the decline of 

 temperature, activity is partially renewed after the drought of summer 

 by the influence of the light early showers of October. The first frosts 

 occur about the middle of November, and the decline into winter is 

 prolonged until the latter part of December. Ice is seldom formed 

 before the beginning of January, and then rarely remains unthawed 

 for 24 consecutive hours. As a physical constant it is a matter of 

 some difficulty to place within 5° of different latitudes isothermal 

 lines for the seasons. That of 60° for the spring, designed for the 

 United States Army Meteorological Register, which connects Sacramento 

 with Beaufort, North Carolina, on the Atlantic coast^ and San Diego, 

 on the Pacific, curves 5° 52' latitude to the south on arriving at 

 the latter point. A corresponding divergence to the north occurs 

 during the winter. The isochimenal line of 45°, which is common to 

 Beaufort, North Carolina, and Sacramento, describes a northerly curve 

 of 8° 03' latitude before reaching the Pacific at Port Orford, Oregon, 

 latitude 42° 44'; the mean annual temperature of which place is only 

 53° 6'. The isotheral of 70°, starting from latitude 40° on the 

 Atlantic side, comes out on the Pacific coast on the parallel of 30°. 

 The great curvature to the south on the Pacific coast during spring 

 and summer demonstrates one of the peculiarities of the distribution 

 of heat in this region. For the mean of the three months of spring 

 the sea temperature which predominates on the line of coast westward 

 of the coast range of mountains is strikingly uniform, and shows but 

 little, if any, advance on that of winter. Indeed, the same may be 

 said of the summer months. For some hundreds of miles on the 40th 

 parallel there is very little difference in the sea temperature for the 

 entire year, and the cold of the Pacific in summer extends, according , 

 to the Army Register, from the 50th to the 30th parallel. Thus 

 while the extremes of summer heat are common to the whole valley 

 of the Sacramento and San Joaquin, the mean summer temperature 

 of San Francisco is only 60°, and there is only one day recorded I 

 among the observations of Dr. Henry Gibbons when the temperature 

 rose above 79° during the summer months. 



