346 
PROFESSOR 0. REYNOLDS AND MR, W. H. MOORBY 
The Adjustment of the Indices on the Barometer. 
48. The lower curtain, together with the slotted tube and cap, is unscrewed from 
the neck of the cast-iron bottle and lifted off over the tube. Then the 30-inch 
standard bar is set on end upright on a surface plate, and the lower curtain, &c., are 
lowered over the bar until the lower lip of the curtain rests on the surface plate, aud 
the top of the bar is 30 inches from this lip. The cap is then screwed dowm until 
light IS seen over the top of the bar through the slot just cut off. Then a vertical 
line, drawn on the cap just above the lijD, at the edge of the scale is the index on the 
cap, and a horizontal line, drawn on the scale level with the lip of the cap, is the 
index point on the scale. And, when these twm lines are brought into this position, 
the distance between the lips will equal the length of the bar. 
In order to check this the curtain is raised, and two thin pieces of chemical paper 
are placed on the surface plate, one on each side of the bar, so as to leave a sj^ace 
between the paper and the bar. Then the curtain is replaced so that it rests on the 
paper, and light can be seen through the interval between the paper and the bar. 
Then light should be seen to an equal extent over the bar, and bj screwing down 
the cap till the light disappears, the thickness of the paper will be measured by the 
angle turned through. 
The construction of this barometer, the first of its kind, v^as undertaken by 
Mr. Foster, who has produced a very beautiful instrument by which direct reading 
can be taken to the ten-thousandth of an inch. The mercury having been re¬ 
evaporated for the purpose in an apparatus belonging to Dr. Schuster by his assistant, 
Mr, S, Stanton. 
This barometer could be used as a pressure gauge for pressure iqj to 34 inches and 
down to 26 inches, and by connecting the mouthpiece with a receiver in connection 
with a mercury or water syphon gauge, with the other limb open to the atmosphere, 
the differences of reading of the barometer for different pressures in the receiv^'er can 
be readily compared with the corresponding differences in the syphon gauge, and by 
such comparisons, taken at intervals till the mercury reaches the closing in of the 
tube, a test is obtained as to the absence of anything but mercury vapour above the 
mercury, 
When the barometer is in connection with the vapour chamber in which the 
thermometer is immersed, the passage of moisture back into the barometer is prevented 
by connecting the tube by a branch with an air receiver, in which the pressure is 
maintained liigher than that in the vapour chamber ; the branch pipe communicating 
with the chamber through a piece of quarter-inch glass pipe, 3 inches long, plugged 
as tightly as possible throughout its length with cotton wool, through which 
the air has to pass from the receiver into the vapour chamber. In this way an 
indefinitely slow current of clean dry air can be maintained into the passage from 
the vapour chamber to the valve which controls the exit of the steam into the 
