PHYSICS OF THE GLOBE. 95 



day and night, during eleven and a half months. These lat- 

 ter observations are, so far as he knows, the only complete 

 ones that have ever been made anywhere for the determina- 

 tion of the daily period of the earth's surface temperature. 

 His discussion of the diurnal variation, with his numerous 

 references to preceding works, constitutes a very important 

 addition to our knowledge. The theory of Poisson gives a 

 very crude approximation to the truth, on account of the dis- 

 turbances introduced by rainfall, air currents, etc. 



By comparing his results for Nukuss with the observations 

 at Melbourne, he finds that both the daily minimum in the 

 temperature of the air and that for the surface of the earth 

 occur almost simultaneously, namely about sunrise; but the 

 maximum occurs in the earth sensibly earlier than in the 

 air. According to the theory, however, the difference should 

 be even greater than is found by observation, so that the 

 maximum temperature on the upper surface of the earth 

 should occur only a short time after midday. In order to 

 reduce observations of the earth's surface for diurnal varia- 

 tion, a table of corrections is given for such combinations of 

 hours as ordinarily occur. In discussing the annual varia- 

 tion of temperature, he shows that the observations of the 

 thermometer lying upon the earth's surface are an important 

 addition to the series, not only on account of their direct 

 practical bearing on vegetation, but especially for their the- 

 oretical bearings, as enabling us to determine the thermal 

 constant. 



The irregular variations in the earth's temperature could 

 scarcely be determined from the five years of observation 

 hitherto treated of; but a parallelism with the air tempera- 

 ture is shown to exist. 



The absolute mean temperatures of the ground are for both 

 places found to be very materially higher than the tempera- 

 ture of the aii*. At 3 meters depth the temperature is at 

 St. Petersburg so much higher than it is at the surface as 

 to show the presence of a large disturbing cause, probably 

 the water of the Neva. The next chapter of Wild's work is 

 an exhaustive collation, discussion, and criticism of all ob- 

 servations of earth temperature made by twenty-two previ- 

 ous observers, which is followed by recommendations as to 

 the best method of determining earth temperatures. His 



