254 Proceedings of Boyal Soeiety of Edinburgh. [sess. 
When the instrument is set, as in fig. 1, the rod DE, hooked to 
the bracket, extends the spring G and keeps the taps BB' closed ; 
but as soon as the messenger strikes the arm F, the bracket E turns 
over, releasing DE. The spring G then contracts, opening the taps 
and admitting water to A, as in fig. 2. The water enters by the 
tap B', through the indiarubber tube B'C, and the air escapes at B. 
Since B and C are close together in the same plane, little or no 
change takes place in the contents of A during hauling up. 
