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XIII. On a Differential Barometer. By the late William Hyde Wollaston, 
M.D. F.R.S. Communicated hy Henry Warburton, Esq. F.R.S. 
Read February 5 , 1829. 
The instrument which I am about to describe, was originally contrived with 
a view to determine the force with which heated air ascends in various kinds of 
chimneys : but since the action of the instrument depends on its rendering ex- 
tremely small differences of barometric pressure discernible and capable of 
measurement with considerable accuracy, it will probably be found applicable 
to a variety of other purposes of more extensive utility. 
In many open fire-places, the slight force with which the smoke ascends, is 
manifest from the facility with which it is forced back by any puff of wind 
that the shutting of a door, or window, or other accidental circumstance may 
occasion : but in some, which are more judiciously or more fortunately con- 
structed, the draft is so considerable as to require a considerable supply of air. 
When the door or windows of a room, in which there is a fire, are open, the 
barometric pressure of the air is not affected by the free current of air that 
supplies the fire ; but if the doors and the windows be all closed, then the ba- 
rometric pressure within the room is lessened by the diminished weight of the 
heated air in the chimney ; and hence the external air presses for admission 
at every crevice of the door or windows, with an energy proportioned to the 
difference of the barometric pressures within and without the chamber. 
If any one were desirous of proving the existence of a difference by a 
mercurial barometer, the instrument employed must be of the very best con- 
struction, and all other circumstances must be very favourable to the experi- 
ment ; otherwise the variation would probably be too small to be perceived, 
although the external pressure might be sufficient to open the door, if only 
closed without being fastened. 
If the pressure were measured by a column of water instead of mercury, the 
