418 

 CORRECTION 



FOR THE HOUR OF THE DAY AND THE SEASON OF THE YEAR AT WHICH THE 

 OBSERVATIONS HAVE BEEN TAKEN. 



IN all the preceding tables, the mean temperature of the layer of air between the 

 two stations is assumed to be given by the half-sum of the temperatures observed at 

 each station, or by . Experience, however, has proved that this assumption is not 

 true under all meteorological circumstances, and that, not to speak of more irregular 

 influences, the temperature expressed by -~ differs in -J- or from the true mean 

 temperature by a quantity which considerably varies with the hour of the day, the 

 season of the year, and the elevation at which the observations are taken. The 

 amount of the correction for the temperature of the air, as given by the various 

 formulas, thus needs to be modified accordingly. In the absence of the data necessary 

 for establishing the law of the decrease of heat on the vertical in the various layers 

 of the atmosphere, at the different periods of the day and of the year, and in differ- 

 ent latitudes, which alone would furnish the means of determining the true value of 

 this correction in these various circumstances, the following empirical tables enable 

 us to form a judgment of the importance of that correction. 



Tables IX. and X. are taken from Berghaus, Grundriss der Geographic, p. 91, 

 and in the Tables accompanying the same work, p. 71. The correction to be applied 

 for the hour of the day at which the observations have been taken, is found by multi- 

 plying the approximate height obtained by the factors in Table IX., giving to the cor- 

 rection the sign of the factor. This table and the following are calculated to be used 

 in the climate of Germany, and for elevations not much exceeding 5,000 feet. The 

 influence of the seasons on the correction is not taken into the account ; judging from 

 Table X., the correction may be, perhaps, too small for the summer months, and may 

 better answer for the autumn. Using these factors, we obtain for the differences of 

 level, in toises, placed at the head of each column, in Table X., the correction cor- 

 responding to each hour, from 6 A. M. to 10 P. M. 



TABLE IX. 



CORRECTION FOR THE HOUR OF THE DAY. 



D 



78 



