112 REPORT— 1867. 



Extreme Monthly Bange. — From Table XIY., \rhich gives the maximum 

 and minimum vapour -pressure and the range for each month, together with 

 their monthly means, it •will be seen that the greatest range in any one 

 month was -384 in March 1864, and the least -149 in May 1863 and Septem- 

 ber 1864, and that January to May inclusive are the months subject to 

 the greatest fluctuation. 



Secular Variation. — An examination of the last two lines in Table XIII. 

 wUl show that, upon the whole, the vapour-pressure has been decreasing since 

 1860. This becomes more evident when we take the means of the results 

 for eveiy two consecutive years. The mean pressure for 1866 was "033 

 below the mean for the seven years, and -061 below the mean for 1860. 

 Looking at the columns which give the monthly means iu each year, we find 

 that the greatest mean monthly pressure was '792 in February 1860, and 

 the least -498 in July 1866. 



Extreme Annual Bange. — Table XV. gives the greatest and least vapour- 

 pressure and the range for each year, with the dates. The mean annual 

 range is -494 inch. 



in. Humidity. 



The degree of humidity is the ratio of the amount of vapour contained iu 

 the air to the amount it would contain if saturated with vapour. Hence, if 

 complete saturation be denoted by 100, and complete dryness by 0, the 

 degree of humidity at any temperature will be obtained by multiplying the 

 actual tension of vapour at that temperature by 100, and dividing the 

 product by the tension required for complete saturation at the same tem- 

 perature. 



Diurnal Variation. — Table XYI. gives the diurnal variation of the rela- 

 tive humidity, so far as it can be directly determined by six-hourly observa- 

 tions. An inspection of the last line but one will show that the humidity is 

 least (67"3) at the warmest observation hour (3| p.m.), and greatest (73-7) 

 at the coldest observation hour (3| a.m.), and that at the other hours it has 

 intermediate values. The mean relative humidity is 70-9, or, complete satu- 

 ration being 100, nearly 71 hundredths. 



Table XYIII., in which the hourly means of the relative humidity, and 

 thcii' deviation from the daily mean, are given, as obtained from four years' 

 term-day observations, shows that the diurnal march, like that of the tem- 

 perature and vapour-pressure, is a single progression, with two branches 

 and two turning- jjoints. In this case, however, the march is in a contrary 

 direction, the greatest humidity occurring at the coldest hours of the day, 

 and the least at the warmest. Thus, the least humirhty (63-6) occurs at 

 1 P.M., from which hour till 2 a.m. it increases to 69-9. It then remains 

 nearly stationary till 8 a.m., showing, however, a tendency to a second 

 minimum at 4 a.m. From 8 a.m. to 1 p.m. it decreases. From 9 a.m. to 

 7 P.M. it is below the mean for the day, and during the other hours above 

 it, attaining its mean daily value about 9 a.m. and 7 p.m. 



Annual Vanation. — The annual march of the relative humidity, and its 

 variation, are shown in the last two columns of Table XYIII. Here we see 

 that February is the most humid month, and November the driest, the 

 mean for the former being 74-7, and for the latter 68-1. From Februaiy 

 to June the humidity decreases ; from June to August it increases ; from 

 Aug-ust to November it decreases again, and from November to February 

 increases. There are thus two maxima and two minima, the February 



