1858.] Meteorological Observations on Parisnath Kill. 13 



temperature for a period of 24 hours, which might be the same for 

 the air and the ground. 



As the atmosphere receives the greater part of its heat not directly 

 from the rays of the sun, but indirectly, from the surface of the 

 ground, the change of the hourly temperatures of the air will follow 

 the extremes of the ground more closely when it is in immediate 

 contact with it, than when further removed from the surface. 

 The temperatures observed at each station as well as the hourly 

 means derived from both, will therefore show a greater daily and 

 hourly variation, than those of the column or stratum of air between 

 them would, if they could be observed. It is only in the means of 

 the whole of the 24 hours, where the extremes of hot and cold com- 

 pensate each other, that the mean temperatures of the stations can 

 be supposed to agree with the real mean temperatures of the air. 



If we assume the mean temperature of the whole day, as taken 

 from the hourly observations at both stations to be the same, or 

 nearly the same, as that which would be obtained from the real 

 hourly mean temperatures of the whole mass of air between them, then 

 the curve of the hourly means of the stations will, in consequence 

 of its greater variation, rise higher above the line of the common 

 mean temperature during the day and sink deeper below it during 

 the night, than the curve of the real hourly mean temperatures. 



These curves must cross each other twice during the 24 hours, 

 once in the morning, and once in the evening. At the moment of 

 crossing the temperature of both is the same, and if the tempera- 

 tures and the pressures be then observed, the resulting height must 

 be the true one. 



To obtain from the results in the table the true height of the 

 temple, it will be necessary to find in the curve of the hourly mean 

 temperatures of the stations, the hours of which the temperature 

 will coincide with the real mean temperature of the column of air 

 between the stations. 



It is at once apparent that this hour cannot be far distant from 

 the hour which will show the mean temperature of the 24 hours, in 

 the present case about 9^ a. m. 



The crossing will take place after 9|-, if the curve of real mean 

 temperatures should reach its daily mean, which I have assumed to 



