-1859] 



WRITINGS OF JOSEPH HENRY. 



179 



From this table we see, first, that the annual mean tem- 

 perature at Paris is 10°-67 C, or 51°-21 F. Second, that the 

 minimum is near four o'clock a. m., and the maximum 

 about two o'clock p. m. Thirdly, which follows from the 

 last, the air is heated during ten consecutive hours, and is 

 cooled during fourteen hours. Fourth, that we fall into a 

 small error in deducing the mean temperature from the max- 

 imum and minimum of the day, the true mean being 10°*67; 

 while the other is 10°."8. Fifth, that the mean temperature 

 is at 8 h. 20 min. in the morning and 8 h. 20 min. in the 

 evening. From this it is evident that, in order to find the 

 mean temperature of the 3'ear, it is sufficient to observe the 

 thermometer each day at twenty minutes past eight in the 

 morning and at twenty minutes past eight in the evening; 

 but if our object is to obtain the mean for each month of 

 the year, it is necessary to change the hour in question, 

 since it is found that for January, 1 o'clock a. m. is the proper 

 hour, for July, 7 o'clock a. m. ; and for all the other months, 

 intermediate hours. The epoch of the mean experiences 

 similar changes in the evening. 



Having discussed the variations of the temperature of 

 different hours, it now remains to speak of the monthly va- 

 riations. From twenty j^ears' observations at Providence,. 

 Rhode Island, the following result has been obtained bv 



