THE SMITHSONIAN INSTITUTION. 



215 



APPENDIX TO MR. RUSSELL'S LECTURES, BY THE SECRETARY OF THE 

 SMITHSONIAN INSTITUTION. 



As an appendix to Mr. Russell's lectures, we give the following tables, 

 showing the mean diurnal variations of the temperature, moisture, 

 pressure, &c., of the air. The principal series are from observations 

 made at Greenwich, near London, under the direction of Mr. Glaishier. 

 The tables for Bombay are from the observations of Dr. Buist, those 

 for Philadelphia are from the valuable series made under the direction 

 of Professor Bache at Girard College. 



By a comparison of the quantities given, it will be seen that all the 

 changes are connected with and depend upon the position of the sun in 

 the heavens, or, in other words, upon the amount of solar heat received 

 at the different hours of the day. The numbers given in the series for 

 Greenwich are deduced from the observations continued for several 

 years, comprising more than 20,000 individual records, and, therefore, 

 abnormal variations are eliminated, and the special changes due to 

 constant causes are exhibited in their true values. 



Table I gives the mean diurnal variation of the temperature of the 

 air in the shade at intervals of two hours, from 2 o'clock in the morning 

 until the end of the 24 hours. It will be seen by this table that on the 

 average the coldest period of the day is a little before sunrise, and that 

 the temperature of the air remains nearly stationary from about 4 

 o'clock until near 6 o'clock in the morning, that it then gradually rises 

 until 2 p. m., when it reaches its maximum, and then declines. The 

 first rays of the sun are probably expended in vaporizing the dew and 

 moisture at the surface of the earth, and as this process renders a large 

 portion of heat latent the air does not increase very rapidly in tempera- 

 ture. 



TABLE I. 



MEAN DIURNAL VARIATION OF TEMPERATURE AT GREENWICH. 



+ 



Minimum. Maximum. 



TABLE II. 



MEAN DIURNAL VARIATION OF THE WEIGHT OF WATER IN A CUBIC FOOT OF AIR AT GREENWICH. 



Table No. II gives the mean diurnal variation of the absolute weight 

 in grains of moisture in a cubic foot of the air, as determined by a 



