326 
Proceedings of the Royal Society of Edinburgh. [Sess. 
rence of this maximum is earlier and a secondary minimum and maximum 
appear, although these are not nearly so well defined as the first. 
The early morning minimum also undergoes a change during the year, 
occurring somewhat earlier in the summer than in the winter. The curve 
for the year is in close agreement as far as form is concerned with that 
obtained at Kew,* Greenwich, f and Kremsmtinster,J although in the case 
of Kew the afternoon minimum is much more pronounced. The curves for 
summer and winter show that there is a decided difference in the type of 
variation in the months included under these classes. For the months 
October to March a well-defined minimum occurs at 4 a.m. and a maximum 
A.M. P.M. 
Fig. 4. 
in the afternoon. In the summer a well-marked minimum occurs at 3 a.m. 
and a maximum at 9 a.m., while there seem to be two other maxima and 
minima much less prominent. 
As the Physical Laboratory is situated in the middle of the town, the 
atmosphere, especially during the day, is likely to be loaded with smoke 
and dust particles, and it is hardly to be expected that the curves showing 
the diurnal variation should be as smooth as those obtained at stations 
where the air is clearer. The disturbance due to soot, etc., would be most 
marked in the latter part of the day, while the effect would be much 
diminished in the early morning. It has, however, been observed at most 
other stations that the smoothest part of the curves occurs in the early 
* Loc. cit. t Greenwich Mag. and Met. Observations. 1 Loc cit. 
