106 The Weather. 



remains longer over these circles than over any other circle of 

 the globe, returns at these times to seek the other tropic at the 

 other side of the equator. In the frigid zones of the globe, 

 within the Arctic and Antarctic circles, the sun remains for 

 days and months together invisible beneath the horizon. To 

 illustrate the intensity of the opposite causes operating in these 

 different regions of the globe, two kinds of thermometers are 

 employed, called solar and terrestrial radiation thermometers. 

 The former has a blackened bulb, surrounded by an exhausted 

 globe of glass, and it occasionally marks 50° F. above the tem- 

 perature of the surrounding air. The latter is an ordinary 

 minimum thermometer, and is exposed at night upon the grass. 

 When exposed on wool or feathers it has been observed by Mr. 

 Glaisher at Greenwich to fall as low as 80° F. below the tem- 

 perature of the air. In both cases the temperature of the air 

 is that of a thermometer in the shade suspended four feet above 

 the ground. This measure in some degree indicates the extent 

 of the causes whose contrast from the equator to the poles pro- 

 duces the prevailing direction of the wind. 



The temperature of the air at the equator is 83° Fahren- 

 heit, nearly without vicissitudes throughout the day or year. 

 In higher latitudes the temperature is not only less, but it is 

 subject to two oscillations, one depending upon the seasons 

 and the other upon the alternation of day and night. The 

 following table exhibits the average value of the first oscilla- 

 tion, at Greenwich, from the mean of fifty years (1814 — 1863), 

 above and below the mean temperature of 49°*03 Fahrenheit. 



Table II. — Annual curve of mean monthly temperatures at 

 Greenwich, for fifty years (1814 — 1863) : — 



Deg. Deg. Deg. 



Jam — 12-1 (F.) 

 Feb. — 10-3 

 Mar. — 7-3 



April— 2-8 



May + 3-9 (F.) 



June +10-1 



July + 12-8 



Auff. + 12-2 



*■&• 



Sep. + 7-6 (F.) 

 Oct. + 1-2 

 Nov. — 5-8 

 i Dec. — 9-2 



The table shows that the seasons are not distributed according 

 to the quarters of the year, but there is a range of nearly 

 25° (F.) between January and July, which are respectively the 

 coldest and the hottest months. December, January, and 

 February compose the Winter, and June, July, and August 

 compose the Summer season. Spring and Autumn occupy 

 the intermediate months. A comparison of the temperature 

 of every day for the same number of years shows that, at 

 Greenwich, the 8th of January and the 15th of July (St. 

 Swithhx's Day) are respectively the coldest and hottest days 

 on the average of fifty years. 



