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Proceedings of the Royal Society of Edinburgh. [Sess. 
line between fogs and clouds. Fogs are generally composed of a greater 
number of smaller particles than clouds ; but cumulus clouds are very much 
like fogs in this respect. I have shown in a previous communication that 
they also are composed of closely packed small particles, and it is only after 
a time that they become fewer and larger by the evaporation of the smaller 
particles and the condensation of the vapour on the larger ones. In these 
phenomena we have a gradual change from haze (the effect of which is 
mainly due to dust and very little to water) to fog (the effect of which 
is mostly due to the water condensed on the dust) and to the obscuring 
effect of cloud (which is almost entirely due to water) ; but between these 
three domains there are no hard-and-fast boundaries. 
The conditions which seemed likely to determine the appearance of the 
morning fogs, if they are a sun effect, had to be sought for in the state of 
the air at the time and place. As all fogs require some water for their 
formation, the humidity of the air had to be observed while they were 
forming ; and as their formation is probably connected with some impurities 
in the atmosphere, observations had to be made also on the direction and 
velocity of the wind, as such records would show whether the air coming 
to the place of observation was pure, or was contaminated with the pollutions 
of the densely inhabited areas in most directions. It was also necessary 
to take the velocity of the wind, as it gives the degree of concentration 
of the impurities. Along with these observations, the transparency of the 
air was noted ; also the amount of cloud on the eastern horizon, to show 
the amount of sunshine. Two observations on transparency were generally 
made — the first one early, generally before sunrise, if possible ; and the 
second at 9.30 a.m., or later in some cases. The transparency of the air 
was observed by noting the amount of haze or fog on a hill 400 feet 
high and about 300 feet above the place of observation, at a distance of 
three-quarters of a mile to the south-south-west. When the air was very 
thick, nearer objects were used for the purpose. From these observations 
on the haze the limit of visibility was calculated. 
Table No. I. gives samples of the observations made for this investiga- 
tion. They are taken from the weather book, which also contains the other 
usual meteorological observations. Only a few of the observations taken 
when the air thickened are given in the table, and it is unnecessary to record 
the great majority of the others, as on most mornings the air did not lose its 
transparency after sunrise. In all those cases in which it did not lose its 
transparency it was found that this was due, either to the absence of sun- 
shine ; to the amount of wind preventing the accumulation of impurities in 
the air ; to the dryness of the air, making it unsuitable for the formation of 
