of the Air and Earth. 55 



under the impression, that the 9 P.M. observation would give 

 an approximation to the nocturnal mean, while the diurnal 

 temperature would result as a mean from those at 7 A.M. and 

 2 P.M. In either case this method might bo best adapted for 

 many climates ; but, as the relative value of the temperatures 

 at times fixed for observation are not constant, but will vary 

 in passing from one climate to another, there is nothing in the 

 method to suit it for universal use. The fact is, it appears to 

 me almost impossible to fix upon any method, except that of 

 hourly observations, which will in every place afford even 

 an approximation to the truth, or obtain harmonious results 

 from elements the relations of which are so variable as those 

 of the temperatures of particular hours. The method which I 

 have adopted, and had in use about three years, I have found 

 experimentally to give results, for this climate, almost in pre- 

 cise accordance with those derived from calculations based upon 

 hourly observations. I have constantly kept daily observations 

 at 6 and 7 A.M., 12 M., 2 p.m., sunset, and 9 P.M., so that I have 

 been able to give my observation any of the common tabular 

 forms, and have always obtained the general results according 

 to each of the methods, and the results have verified my previ- 

 ous conclusions by varying from each other as follows : The 

 mean annual temperature of this place (Jackson, La., U. S.),by 

 my method, is G4.24°, Fahrenheit ; as derived from the sunrise, 

 2 P.M., and sunset observations, 66.30° ; and, during the same 

 period, the mean obtained from the 7 A.M., 2 P.M., and 9 P.M. 

 observations is 65.62°. In this way I have compared results 

 by each method with each other, and with those obtained from 

 hourly observations. I have found that the following method 

 gives for this place results more accurate than any other I 

 have been able to derive. The mean temperature of the diur- 

 nal portion of the twenty-four hours is derived as the mean of 

 the highest and lowest temperatures of that portion ; that is, 

 of the sunrise observations and that of 2 P.M. The nocturnal 

 mean will, in like manner, result as the mean of the highest 

 and lowest temperatures of that portion ; that is, of the sun- 

 set and sunrise observations. The average of these two means 

 will give the mean for the twenty-four hours. By this method, 

 the sunrise temperature being the lowest for the twenty-four 



