110 REPORT— 1867. 



13°, in ITarcli 1866, and tlie least of the extreme diurnal ranges G"^-i, in 

 October and jSTovember 1862, and that the greatest variations of temperature 

 take place diu'ing the summer months, namely, from November to May. 



Least Diurnal Range. — The least range of temperatiu-e, on any one day, 

 during the same five years, was l°-4 in June 1866, and the greatest of the 

 least diurnal ranges 6° in January 1865, as appears from Table lY., "which 

 likewise sho'ws that the summer months are subject to greater fluctuations 

 of temperatiu'e than the svinter months. 



Mean Diurnal Range. — Table V., in which the mean diurnal range for 

 each month is given, shows that the mean diurnal range for the year is 

 6°-69, and that the greatest fluctuations occiu- in the summer months. 



Annual Variation. — The annual march of the temperature, derived from 

 the daily six-hourly observations for seven j-ears, is exhibited in the last 

 column but one of Table YI. Like the diurnal march it is a simple progres- 

 sion, having one ascending and one descending branch. The least mean 

 monthly temperature is 71°'95 for July, and the greatest 81°-72 for Januaiy. 

 From July to January the temperature increases, and from January to July 

 it decreases. This progression, as is well known, depends on the motion of 

 the earth in its orbit. The epochs of highest and lowest temperature, how- 

 ever, do not coincide with those of the sun's highest and lowest meridional 

 altitudes, but occur at later periods, the maximum temperature about the 

 4th of February, and the minimum about the 7th of August. The last 

 column shows the amount of variation, or the excess and defect of the mean 

 temperature (77°"11) on the monthly means. During the six summer 

 months the temperatiu'e is above, and dui-ing the si^c winter months below, 

 the mean temperature, the epochs of which are the 7th of Maj- and the 5th 

 of November. 



For the sake of comparison, I have given in Table YII. the mean monthly 

 temperature obtained by taking the mean of the daily readings of the maxi- 

 mum and minimum thermometers. The mean annual temperature thus 

 derived is 77°-80, or 0-69 higher than that given by the six-hourly obser- 

 vations. 



Temperature in the Sun's Rags. — Table YIII. shows the mean monthly 

 maximum temperature in the sun's rays, obtained by daily observation of a 

 black bulb thermometer inclosed in an exhausted tube, exposed at an eleva- 

 tion of 40 feet above the ground, and protected, as far as jiossible, from local 

 radiation. The results, which, as measures of solar radiation, are, of course, 

 subject to the usual objections, present a progression similar to that of the 

 temperature in the shade, the greatest mean monthly maximum being 117°"6 

 for January, and the least 101°-2 for July, — the progression being harmo- 

 nious, except that the temperature in February is somewhat lower than in 

 March, o-wing probably to the former month being cloudier than the latter. 



E.vtreme Monthly Eange. — Table IX. shows the maximum and minimum 

 temperature and the extreme range of temperature for each month, and 

 their monthly and yearly means. The greatest range in any one month was 

 18° in November 1864, and the least 6°-4 in October 1862. The greatest 

 fluctuations occur in the warmest months. 



Secular Variation. — The last line in Table YI. shows that the tempera- 

 ture has varied little, it being for foru* years out of the seven almost the same, 

 and the greatest difference between any two years being only 0°-95. 



A similar remark applies to the numbers in the last line in Table YII., 

 showing the mean annual temperature as deiived from the self-registering 

 thermometers. 



