1893.] of Observations of Air Jemperature and Pressure. 73 



ing figures in some degree comparable with those derived from 

 actual observation, I have assumed (following the empirical formula 

 before given) the power of a vertical sun to be 10. Having calcu- 

 lated the sun's altitude for each hour of the day, for midwinter, the 

 equinox, and midsummer, for certain selected latitudes, the corre- 

 sponding heating effects have been computed, to which the usual 

 method of analysis having been applied, the following results are 

 obtained (pp. 74 and 75). 



These figures represent the values of the components at midnight. 

 The signs indicate that the maximum of the first component is in all 

 cases at 180°, or noon ; of the second component the maximum in all 

 cases it at 0°, or midnight; of the third component in all latitudes 

 the maximum in the winter is at 60°, or 4 a.m., in the summer at 0°, 

 or midnight, the change taking place at the equinox, when the com- 

 ponent becomes zero ; of the fourth component in the lower latitudes 

 up to 40°, the maximum is at 45°, or 3 a.m., at all seasons ; in the 

 higher latitudes the maximum is at 0°, or midnight, in winter and 

 summer, and at 45°, or 3 a.m., at the equinox. 



For comparison, the following results from actual observations at 

 latitudes specified are also given in similar form. 



The close correspondence of the main features of these two tables is 

 obvious. 



The conclusion is unavoidable, that, although both in the actual 

 and hypothetical cases the harmonic components when combined are 

 truly representative of the peculiar forms of the curves from which 

 they were derived, this affords no evidence of the existence of recur- 

 ring cycles of action corresponding to the different components, but 

 that the results are, to a great extent, due to the form of the 

 analysis. 



The diurnal curve of temperature is not symmetrical in relation to 

 the mean value, the maximum day temperature being much more in 

 excess than the minimum night temperature is in defect. To adjust 

 the first component, which is symmetrical about its mean value, to 

 the actual unsymmetrical curve, it must be modified by the other 

 components. That of the second order which has one of its maxima 

 not far removed from the minimum of the first order supplies the 

 chief portion of the compensation due to this cause. 



Further, from the character of the analysis, when the diurnal curve 

 is symmetrical on either side of the hour half-way between noon and 

 midnight, that is, when the day and night are equal in length, the third 

 component becomes zero. Any departure from this symmetry intro- 

 duces a component of the third order, with the result that with a day 

 shorter than 12 hours one maximum will fall in the day between 

 6 a.m. and 6 p.m., and the other two in the night between 6 p.m. and 

 6 a.m. ; while with a day longer than 12 hours, two maxima will 



