OF THE ATMOSPHERIC TEMPERATURE. 181 



the autumnal equinox. The average deviation from this date is 4 days, earlier or 

 later. 



The epochs of the mean value of the year, reached with rising temperature, are 

 comprised, with the exception of lUoolook which is doubtful, between April 11, at 

 Fort Brown, Texas, and May 9, at Sitka; the average date for all other stations 

 being April 2 1 , which is 32 days after the vernal equinox. The average deviation 

 from this date is 5 days, earlier or later. 



The dates for the maximum temperature, with the exception of that for San 

 Francisco which is anomalous and delayed to Sept. 23, are comprised between the 

 limits of July 8, at Van Rensselaer Harbor, and August 15, at San Diego ; all 

 the other stations cluster about July 24, which is 33 days after the summer 

 solstice. The average deviation from this date is 4| days, earlier or later. 



The dates for the minimum temperature vary between the limits of December 

 31, at New Orleans and at Fort Craig, and February 16, at Port Foulke ; we have 

 to except, however, the date for Van Rensselaer Harbor, which has the highly 

 uncertain date March 1 ; the remainder of the stations cluster about January 18, 

 which is 28 days after the winter solstice. The average deviation from this date 

 is 6 days, earlier or later. 



We thus see that the daily balance between the decreasing radiation and the 

 increasing insolation at the midwinter extreme is struck earlier by 5 clays than 

 the opposite balance between the decreasing insolation and the increasing radia- 

 tion at the midsummer extreme, as compared with the corresponding astronomical 

 epochs. 



Altogether, then, the curve expressive of the annual distribution of heat, for our 

 stations, follows in epoch, on the average 31 days, or very nearly yV of a year, the 

 corresponding astronomical epochs depending on the revolution of the earth around 

 the sun. 



Examining the dates of the four epochs with respect to geographical distribution 

 of stations within the area of the United States, we find for the 9 Atlantic coast 

 stations, Nos. 13, 15, 19, 21, 24, 27, 30, 34, 36, the average dates: July 25, January 

 17, for maximum and minimum, and April 23, October 24, for an average of the 

 year in spring and autumn. Compared with the normal epochs, viz.: — 



July 24, January 18, April 21, and October 22, they appear about 1 day later 

 than the normals. No dependence on the latitude is indicated. 



The 10 centrally located stations in the valley of the Mississippi and east of the 

 foot of the Rocky Mountains, also including two Texas stations, viz. : Nos. 37, 12, 

 20, 23, 26, 28, 29, 32, 33, and 35, give the respective dates:— 



July 23, January 12, April 14, and October 19, which are on the average 4 days 

 earlier than the normal values. The latitude of the stations is apparently of no 

 consequence in this inquiry. Similarly we find for the three Pacific coast stations, 

 Nos. 9, 39, and 46 the respective dates : August 10, January 22, May 4, and 

 November 1, which are on the average 15 days later than the respective normal 

 values, while at San Francisco the dates for the maximum and for the autumnal 

 mean are still later. With respect to the annual thermal epochs we thus notice 

 the apparent effect on the coast stations by the Atlantic is to retard them by about 



