thermometrical barometer for measuring altitudes. 189 
height of the mercurial thread, and by having no lateral 
motion, confines the view to the same direction and prevents 
parallax. 
In boiling, the bulb should be exposed to steam only, as 
being steadier in its heat than the water. My boiler is a tin 
cylinder 5,5 deep, 1,2 diameter, with an external cylinder 
1,4 diameter for preventing the transmission of heat, the 
bottom only is single. The interior cylinder has a brass 
collar soldered into it, having an internal screw which fits to 
either of the external screws on the plate Fig. 3. ; so that 
what is boiler when fixed below plate 3, becomes a case to 
protect the scale, when screwed to the upper side of that 
plate. The top of the external cylinder being closed into the 
same brass collar, becomes slightly conical, and is soldered to 
it. An opening of 0.2 diameter is made through both cylin- 
ders, immediately under the collar, as a vent for the steam 
from within, but is prevented from communicating with the 
annular space between the vessels, lest inconvenience should 
arise from water accidentally getting between them. 
Another tin cylinder, 1,2 in diameter, and 2,1 deep, with a 
similar screw collar at top, forms a case for protecting the 
bulb when screwed to the under side of plate 3, and is also 
a measure for the quantity of water put into the boiler, which 
should not touch thj bulb; it is here 1,25 below it. 
For the purpose of rendering every thing requisite for use 
portable also with the thermometer, I made a stand for it, 
which is convenient and will readily be described. Round 
the outside of the boiler, and just below the conical closing of 
it, is soldered a ring of brass wire ST. Fitting to the conical 
top of the boiler is made another short cone of thick tin, which 
