66 
Anderson’s New Atmometer, 
above must at length counteract this process, it is still sulRcient 
to render the indications of the instrument altogether uncertain. 
As a proof of this, we shall mention one instance among seve- 
ral others that might be stated. On an evening of a dry sum- 
mer day the water in the tube of the atmometer stood at 100 ; 
during the night there was a heavy fall of rain, and next morn- 
ing the surface of the liquid was as high as 15.. This fact we 
apprehend is decisive as to the merits of the instrument. It is 
therefore totally unfit for measuring the quantity of evaporation 
during an interval of any considerable length. 
The atmometer recently invented by Mr Anderson, is un- 
doubtedly the simplest as well as the most ingenious instrument 
of the kind hitherto proposed. It consists of a bent glass tube 
ABCDEF (Plate I. Fig. 5.), of sufficient width to admit of a 
liquid moving easily from one part to another, and swelling out 
into the bulbs BC and EF. Into this tube at A is introduced 
a quantity of alcohol, which, after being conveyed into the bulb 
or wider tube EF, is thrown into a state of ebullition, and while 
the steam is issuing from A, the tube is there hermetically sealed, 
so tliat the air is completely expelled from the space ABODE. 
The bulb EC is then covered with moistened silk or paper, and the 
instrument freely exposed. In consequence of the pressure of the 
air being removed from the surface of the alcohol in the bulb EF, 
a portion of that liquid passes into vapour, and occupies the 
empty part of the tube. Were the whole of the instrument at 
the same temperature, this process indeed would quickly be 
stopped by the pressure of the vapour itself on the surface of 
the altohol;. but as the bulb BC has its temperature reduced 
by the external evaporation from the moistened silk or paper, 
the vapour which rises from EF is there condensed, and runs 
down in a liquid state into the tube AB. This distillation goes 
on more or less rapidly, according to the degree of cold induced 
upon the bulb BC, that is, in proportion to the external evapo- 
ration ; and, consequently, the quantity of liquid collected in 
the tube AB, is a measure of that evaporation. When the at- 
mosphere is completely saturated with moisture, or when the 
evaporation ceases, the temperature of AB will be the same as 
that of any other part of the tube, and the distillation, therefore, 
for the reason already stated,, will also cease^ 
