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_ suggested the probability that the forenoon and evening maxima result more 
or less from the presence of aqueous vapour, either in an invisible or con- 
_ densed state. 
_ $8. With a view to submit this notion to the test of observation, an attempt 
_ has been made (it must be confessed of a very rough and arbitrary character) 
to separate the high from the low tensions; the point 60 div. of Volta’s elec- 
trometer No. 1 has been provisionally assumed as the separating point, and 
~ all tensions above it have been regarded as high, those below it the converse. 
~ The result of this separation, so far as the low tensions are concerned, has 
been to exhibit series of curves, those of the summer and entire year being 
-somewhat in harmony with the aggregate curves for the same periods; the 
~ forenoon and evening maxima however are greatly subdued, but still the 
_ evening holds the most prominent position. The curves of the entire year 
suggest the probability that a single progression would be obtained on the 
removal of the two maxima. 
_ 4, The winter curves of low tension strongly confirm this suggestion. The 
“approach to a single progression is very apparent in the winters of 1845 and 
1847; the mean curve however still presents the two maxima, although their 
-altitudes are considerably more equal in value than any of the curves yet 
_ contemplated ; their interval in time (6 hours) is also less than most of the 
others, especially the aggregate curves, the most usual interval of these being 
~ 12 hours, 
' 5. The salient points characterizing the two series of curves (aggregate 
~ and low tension) are a decided development of the forenoon and evening 
maxima in the aggregate, and a considerable subduing of these features with 
an approach to a single progression in the low. 
_ Diurnal period above 60 div., Year.—We are now prepared to enter on the 
“discussion of the high tensions, with the expectation that the two maxima so 
_ prominently developed in the aggregate curves will form very decided fea- 
_ tures in those deduced from observations above 60 div. It is necessary to 
_ observe here, that the observations above 60 div. will not furnish the entire 
diurnal march of the high tensions, none being recorded at the hours of mid- 
night and 2 a.m.; very few indeed are entered at 4 a.m.; and those finding 
~ entrance at 6 a.m. being mostly confined to the summer half-year, the diur- 
~ nal march cannot be accurately said to commence until 8 a.m. In the fol- 
- lowing tables and curves, with the exception of those having reference to the 
_ summer half-year, the diurnal march is given between 8 a.m. and 10 P.M. 
_ inclusive; in the summer it commences two hours eatlier. 
__ The 2647 high readings during the three years are thus divided among the 
twelve observation-hours :— 
ms Taste XXXVIII. 
_ Number of positive readings above 60 div. at each observation-hour in the 
three years 1845, 1846 and 1847. 
| — | | | |__| —_—_—$ | —_———_ | —— | —-— | | —— | qq“ \— 
o: 18 73| 75 | 87 | 107 | 130] 164 | 852 
| 1846.)......}.....] 2 20 | 118} 124 76 | 83 | 86; 110] 138 | 157 | 914 
1 1847,)......| ...... 26 | 124 72} 73 | 80] 96] 129} 152 | 881 
64 | 334 221 | 231 | 253 | 313 | 397 | 473 | 2647 






In connection with this table it will be observed that it furnishes two 
