72 



TRANS. OP THE ACAD. OF SCIENCE. 



Thus the law, found in 1859, is confirmed by another year's 

 observation : 



The difference is greatest at 9 P.M., less so at 7 A.M., and 

 least at 2 P. M. 



I have no doubt that observations, made at sunrise, would 

 prove the difference of temperature at that time of the day 

 to be greater than at any other hour. With the rising of the 

 sun the difference steadily diminishes until the temperature 

 of the day has reached its maximum, when it increases again 

 through the evening and night to the period of minimum 

 temperature at sunrise. 



The mean difference of temperature in city and country 

 was found, in 1859, to be a little less than the mean of the 7 

 o'clock observations ; in 1861 it was equal to the same. The 

 differences in both years are greater in the summer and fall 

 months, less in the winter and spring months. 



The last columns of the table show that the minimum tem- 

 peratures (at least, at the hours of observation) are always 

 lower in the country than in the city, to the amount, on an 

 average, of 3.6 degrees. The maxima are usually, but not 

 always, lower in the country, but the difference is on an av- 

 erage, only, of 1.5 degrees; and the highest temperature, 

 during the very hot weather in the beginning of August, was 

 found higher in the country than in the city. 



Another interesting result is obtained by comparing both 

 sets of observations in regard to the difference of Evapora- 

 tion and of Relative Humidity in city and country. The 

 amount of evaporation is indicated by the difference of the 

 dry and wet bulb thermometers. Mr. Fendler's wet bulb 

 thermometer was generally not as much below my wet bulb 

 as his dry bulb was below mine, consequently the difference 

 was almost always less at his station than at mine ; three 

 times it was equal, and twice (at 7 A.M. in January and in 

 December) it was even a little greater in the country. 



The average results for the year may be summed up thus : 



The following table exhibits the difference of Evaporation 

 and Relative Humidity, at the different hours of observation, 

 in each month of the year: 



