354 



Wm. A. Norton on the Variations 



of temperature of two places thus situated. The theory strictly 

 requires that the difference between the average temperatures, at 

 any time, of the ground near the surface should be taken, but 

 there is little reason to doubt that the laws of the variations of 

 this difference, at any one season, will be very nearly the same 

 as of the variations of the difference of surface temperatures. 

 Whatever errors may result from taking the latter difference in- 

 stead of the former will probably be simply errors of quantity. 



Although I am not in possession of the precise data demanded 

 for a minute prosecution of the present inquiry, still the meteor- 

 ological observations made at Philadelphia and Washington will 

 furnish differences of temperature, which will doubtless, in the 

 average for weeks and months, differ little from the differences 

 demanded. Let us then compare the curves showing the mean 

 diurnal variations of the vertical intensity for the different quarters 

 of any one year, 1841 for example, with the curves showing the 

 mean diurnal variations of the difference of temperature of Wash- 

 ington and Philadelphia for the same periods of time. (See figs. 

 13 to 20.) On examining these curves it will be seen that the 

 maximum of vertical intensity, at all seasons of the year, is not 

 far from the hour of maximum daily temperature (between noon 

 and 4 p.m.) and the minimum toward midnight, and that the 

 same is true of the differences of temperature. The curves of 

 vertical intensity, for the other years, conform to the same general 

 law, and the calculations of difference of temperature, so far as 



Curves showing the Mean Diurnal Variations of the Vertical Force, for quarters 



of years. 



Fig. 13. 



Jan., Feb. and March, 1842. 



Fig 14. 



April, May and June, 1841. 



