“It differs from barometers hitherto made in points of detail, rather than principle:—1. The glass tube is packed with vulcanised india-rubber, which checks vibration from concussion; but does not hold it rigidly, or prevent expansion. 2. It does not oscillate (or pump), though extremely sensitive. 3. The scale is porcelain, very legible, and not liable to change. 4. There is no iron anywhere (to rust). 5. Every part can be unscrewed, examined, or cleaned, by any careful person. 6. There is a spare tube, fixed in a cistern, filled with boiled mercury, and marked for adjustment in this, or any similar instrument.
“These barometers are graduated to hundredths, and they will be found accurate to that degree, namely the second decimal of an inch.
“They are packed with vulcanised caoutchouc, in order that (by this, and by a peculiar strength of glass tube) guns may be fired near these instruments without causing injury to them by ordinary concussion.
“It is hoped that all such instruments, for the public service at sea, will be quite similar, so that any spare tube will fit any barometer.
Fig. 16.
“To Shift a Tube.—Incline the barometer slowly, and then take it down, after allowing the mercury to fill the upper part. Lay the instrument on a table, unscrew the outer cap at the joining just below the cistern swell, then unscrew the tube and cistern, by turning the cistern gently, against the sun, or to the left, and draw out the tube very carefully without bending it in the least, turning it a little, if required, as moved. Then insert the new tube very cautiously, screw in, and adjust to the diamond-cut mark for 27 inches. Attach the cap, and suspend the barometer for use.
“If the mercury does not immediately quit the top of the tube, tap the cistern end rather sharply. In a well-boiled tube, with a good vacuum, the mercury hangs, at times, so adhesively as to deceive, by causing a supposition of some defect.
“In about ten minutes the mercurial column should be nearly right; but as local temperature affects the brass, as well as the mercury, slowly and unequally, it may be well to defer any exact comparisons with other instruments for some few hours.”
Messrs. Negretti and Zambra are the makers of these barometers for the Royal Navy. Fig. 16 is an illustration.