-1859] WRITINGS OP JOSEPH HENRY. 181 



in the two decades scarcely differ a degree in the whole 

 series. If the times were further extended the agreements 

 would probably be closer, the instruments remaining the 

 same.. These facts illustrate the truth of what we have pre- 

 viously said relative to the deduction of definite results from 

 the most complex and variable elements, and the perman- 

 ency of the mean temperature of a given position ; the sum 

 of the variations consisting in oscillations on either side of 

 the mean, which in the aggregate neutralize each other. 



It is known from extended observation that the same 

 weather exists at the same time over a large extent of country. 

 For example, during a cold winter, it is comparatively cold 

 over the whole of France; and in the State of New York, 

 though the temperature be different in different places, a 

 cold January will be cold over the whole state; hence a table 

 carefully made at any one place will serve to indicate the 

 relative temperature of others in the same district. 



We see from the foregoing table that the greatest heat of 

 the day at Paris happens at 2 o'clock, while we know that 

 the solar rays are most intense at 12 o'clock. We have in 

 a previous report given an explanation of this phenomenon, 

 namely, that the earth is constantly radiating heat into space 

 and receiving it from the sun the whole time it is above the 

 horizon; the temperature therefore will constantly increase 

 while the amount of heat received is greater than that given 

 off. The greatest amount of heat received in a minute is at 

 12 o'clock, and hence the increase of temperature at this 

 time will be the greatest; but the earth after 12 o'clock still 

 continues to receive more heat than it gives off, and hence 

 the temperature of the air will still continue to increase, 

 though at a less rapid rate, until about 3 (/clock in our lati- 

 tude. The radiation into space from the earth and the absorp- 

 tion from the sun about balance each other, and the tem- 

 perature will then remain stationary at its maximum point 

 during some time, the loss and gain being equal. After this 

 the loss is greater than the gain, and this goes on continually 

 until the setting of the sun, when the radiation is entirely 

 uncompensated and cooling takes place, at first with a sudden 



