394 
Proceedings of the Royal Society of Edinburgh. [Sess.. 
Six’s thermometer. It consists, as usual, of an inverted bulb G, filled with an 
organic liquid (in this case having very uniform expansion); a U-tube H, 
the bend of which contains mercury ; and the bulb I, partly filled with liquid. 
When the temperature rises, the mercury in the inner limb of the U-tube is 
depressed by the expansion of the liquid in the bulb G. Now it is possible, 
by proper adjustment of the bulbs A and G and of the stems B and H, to 
make the expansion of the liquid along H equal to that of the air along B,, 
under the constant pressure of 80 cms., for equal rises of temperature ; and 
not only so, but the level of the mercury in H can be made the same as that 
of the mercury in B at 0° C. When these adjustments are made once by 
the instrument-maker, then all that is needed at any field temperature to 
bring the air in A to the required standard pressure is to work the cistern 
screw till the mercury in the stem B comes to precisely the same height as 
the mercury in the inner tube of the Six’s thermometer. The mark 80 on 
the scale is then brought to this same point, and then the atmospheric 
pressure can be read off directly by looking at the top of the mercury in E. 
To facilitate the accurate adjustment of the menisci in H and B to the 
same level, I propose to attach to the scale a small blackened plate with 
sharp, straight lower edge, exactly on the level of the 80-cm. mark. This 
plate lies behind the stems B and H, which have otherwise a transparent 
background. It will first be brought down to touch the meniscus of the 
Six’s therometer, the scale being thus at the same time put in proper 
position ; the mercury in B is then brought up to its lower edge ; and finally 
the reading of the atmospheric pressure is taken. In instruments where 
high accuracy is aimed at, a small reading microscope travelling up and 
down along the front of the instrument will be provided, to be used both 
for the accurate adjustment of the mercury in B and for the accurate read- 
ing of the mercury in E. 
It is absolutely necessary that the instrument should always retain the 
air in the bulb A and stem B in whatever position it may be carried. To 
effect this a short part of the bore of B at its lower end is made capillary,, 
and the end of the stem is ground flat and can be closed by a small screw 
working from below. This closing must always be done when the instru- 
ment is to be carried. The upper end of E also has a small screw-cap, and 
an arrangement is proposed for the removal from the cistern C of any air 
which may lodge in it from the stem E. 
The preliminary adjustment of the bulb A so that the air thermometer 
and Six’s thermometer may have degrees of exactly equal length is the 
most difficult point in connection with the instrument. But it is to be 
noted that after the first rough adjustment to equality is made, a verjr 
