of Edinburgh, Session 1884-85. 
117 
the bulb of the instrument a large beaker lined with moist filter- 
paper, that the arrangement can be made extremely sensitive. The 
mercury surface is seen to become flattened the moment the beaker 
is applied, and a few minutes suffice to give a large descent, pro- 
vided the section of the tube be small, compared with the surface of 
the ball. 
I propose to employ the instrument in this peculiarly sensitive 
state for the purpose of estimating the amount of moisture in the 
air, when there is considerable humidity ; but in its old form when 
the air is very dry. For this purpose the end of the tube of the 
atmometer is to be connected, by a flexible tube, with a cylindrical 
glass vessel, both containing mercury. When a determination is to 
be made in moist air the cylindrical vessel is to be lowered till the 
difference of levels of the mercury amounts to (say) 25 inches, and 
the diminution of this difference in a definite time is to be carefully 
measured, the atmospheric temperature being observed. On the 
other hand, if the air be dry, the difference of levels is to be made 
nil , or even negative, at starting, in order to promote evaporation. 
From these data, along with the constant of the instrument (which 
must be determined for each clay ball by special experiments), the 
amount of vapour in the air is readily calculated. Other modes of 
observation with this instrument readily suggest themselves, and 
trials, such as it is proposed to make at the Ben Nevis Observatory 
during summer, can alone decide which should be preferred. 
Monday, 2 d March 1885. 
BOBEBT GBAY, Esq., Vice-President, in the Chair, 
At the request of the Council, an Address on the Becent Progress 
of the Geological Survey of the United Kingdom was given by 
Professor Archibald Geikie, F.B.S., Director of the Survey, 
On the motion of the Chairman, a vote of thanks was accorded 
to Professor Geikie for his Address. 
PRIVATE BUSINESS. 
The following Candidates were balloted for and declared duly 
elected Fellows of the Society: — Professor Elgar, Dr Orme Masson, 
and Dr J. M. Macfarlane. 
