"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. "_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. [Illustration: Fig. 16.]