260 Proceedings of Royal Society of Edinburgh. [sess. 
the fog or not, that is, for determining whether it is a wet or a dry 
fog. This instrument can also be used for counting the number of 
drops which fall on a given area in a given time. It might he 
thought it would he quite unnecessary to use an instrument for 
telling us whether there are any water drops in the fog or not ; 
because if there are any drops in it, they will be falling, and will wet 
all exposed surfaces, so that a piece of mirror would be all that would 
be necessary for the purpose. Such, however, is not the case. I 
have found in many fogs when all exposed surfaces were quite dry, 
that there were great quantities of water drops in the air and falling 
on all exposed surfaces. These drops, however, are so extremely 
small they are invisible under ordinary conditions, and being so 
small they rapidly evaporate, as all exposed surfaces are generally 
more or less heated by radiation during the day. 
The plan I have adopted for observing these drops is the same as 
that used for observing the artificially made drops in the “ Pocket 
Dust-Counter,” described in a previous communication.* The new 
instrument consists of a glass micrometer divided into squares of a 
known size, a spot-mirror for illuminating the stage, and a strong 
lens or a microscope for observing the drops on the stages. The 
space between the micrometer and the lens is open, so that the air 
passes freely over the stage, and the drops that fall on its surface 
are easily seen. These drops are very small ; as yet I have not had 
an opportunity of measuring them. They of course vary greatly in 
size, and many of them even when spread on the glass are not more 
than 0*05 mm. In observing these drops the attention requires to 
be confined to a limited area of the stage, as many of the drops 
rapidly evaporate, some almost as soon as they touch the glass, 
whilst the larger ones remain a few seconds. A square of 1 mm. is 
rather small an area, but one of Jg-, or ^ cm. does very well when 
working with a magnifying lens. 
The following are samples of the results obtained when working 
with this instrument. On the 19 th February 1891, at 10 a.m., 
the fog was so thick that objects beyond 100 yards were quite 
invisible. The surfaces of bodies exposed in the open air were 
dry. On this occasion the number of drops falling per minute 
varied greatly from time to time. The highest number observed 
* Proc. Roy. Soc. Edin. , vol. xviii. 
