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New Form of Mountain or other Barometer. 235 
finally a clamp-screw, whose head is seen in the sketch, working 
through the jaws aforesaid, completes the arrangement, and al- 
lows of its being put on or taken off at pleasure. It is not advi- 
sable to interpose between’ the spring and the glass any paper, 
woollen cloth, or other hygrometric or electric substance. 
4. The tube itself; as to its contents and their manner of in- 
troduction, I will speak hereafter. The tube should be about 
35 inches long ; in the sketch, in order to bring it within the lim- 
its of the page, I have represented it as much shorter. The 
open end is ground off as smoothly and as nearly at right angles 
to the tube-axis as may be; about an inch and a half from the 
end, a short line is marked to serve as the zero of the graduation ; 
and at a half-inch or some other convenient length above, another 
mark is made for the subsequent adjustment of the float when the 
instrument is in use. : ; | 
‘The ‘graduation is then continued, either in the engine or by 
hand, so as to lay off in tenths of inches from 27 to 31 inches ; 
and higher or lower in the discretion of the maker, on the tube. 
It is obvious that this division may, if so desired, be carried to 
hundredths in the same operation: but I have not found it de- 
sirable. Many tubes are sufficiently soft to be cut at once by a 
hard steel point in the engine: if the cutter be made of white 
cast iron of a kind frequently met with, hardly any tube will pre- 
sent a difficulty ; but in any event, and in the particular case from 
which this description is taken, the resort to fluo-silicic ‘acid to 
etch the lines is ready and successful. In whatever manner 
produced, the reflections from the mercury column behind them 
lend an increased depth and beauty. = 
5. The vernier is composed of a piece of watch-spring long 
enough to reach round three fourths of the circumference of the 
tube, and thus to hold in its place the scale, riveted at right angles 
to the spring. ‘The lower edge of the spring is worked true ; and 
forms an index-edge, similar in fact to what. is generally used in 
mountain-barometers. The scale itself is made of mother of pearl ; 
not so much from the luxury of taste, as for two practical reasons : 
the first, that the peculiar surface of pearl offers much less friction 
than any other material otherwise applicable, and thus makes the 
sliding of the vernier very uniform, easy and pleasant; and the 
second, that the transparency of the pearl allows the graduation 
of the tube to be seen through the scale of the vernier, and the 
ris 
