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effective in producing the change. These term observations should be separately registered under that head.*

9. Occasional hourly series of observations may be made with advantage under several circumstances, as, for instance-1stly. When becalmed for any length of time, especially when near the equator, with a view to determining the laws and epochal hours of diurnal periodicity. 2ndly. When a party leaves the ship, furnished with a portable barometer or other instruments,† for the measurement of heights of mountains, or with other objects. 3rdly. During threatening weather, and especially during the continuance of gales, and for some time after their subsidence, as will be more particularly specified under the head of "Storm Observations." 4thly. In certain specified localities mentioned in the next article by Mr. Birt. 5thly. Whenever a continued rise or fall of the barometer has been noticed as at all remarkable, it should be pursued up to and past the turn, so as to secure the maximum elevation or depression, and the precise time of its occurrence; and a register of such maxima or minima should be kept distinct from the regular entries.

Of Meteorological Instruments; and first, of the Barometer and its attached Thermometer.

The barometer on shipboard should be suspended on a gimbal frame, which ought not to swing too freely, but rather so as to deaden oscillations by some degree of friction. Before suspending it, it should be carefully examined

*The term observations have in a great measure fallen into disuse, a large mass of them having accumulated which require reduction and discussion. Before undergoing the trouble of making them, therefore, in any particular case, the observer would do well to secure co-operation from officers in other ships, or to inquire at what fixed stations they continue to be made.-R. M.

The aneroid, if used in excursions of this kind, should be first accurately compared with the standard barometer; after which its indications would seem, from recorded trials, to be tolerably dependable as far as 2000 feet in altitude. The comparison should be repeated on returning.

for air-bubbles in the tube and for air in the upper part above the mercury, by inspection, and by inclining the instrument from the vertical position rather suddenly till the mercury rises to the top with a slight jerk, when, if it do not tap sharp, the vacuum is imperfect; and if the sound be puffy and dead, or is not heard at all, air exists to an objectionable extent, and must be got rid of by inversion and gently striking with the hand to drive the bubble up into the cistern. The lower end of the tube, which plunges into the cistern in well-constructed marine barometers, is contracted so as to diminish the amount of oscillation produced by the ship's motion. The instrument should be suspended out of the reach of sunshine, but in a good light for reading, as near midships, and in a place as little liable to sudden changes of temperature and gusts of wind as possible. The light should have access to the back of the tube, so as to allow the index to be set with its lower edge forming a tangent to the convex surface of the mercury. In well-constructed barometers the slider has its lower part tubular, embracing the tube, and can be made to descend by the rack-motion of the vernier till it becomes an upper tangent to the mercury the eye being on its exact level, a reflected light by day, or white paper strongly illuminated from behind at night, will throw the light properly for setting the vernier correctly. The exact height of the cistern above the ship's water-line should be ascertained and entered on the register.

The attached thermometer ought to indicate a temperature the exact mean of that of the whole barometric column. Its bulb, therefore, ought to be (though it seldom is) so situated as to afford the best chance of its doing so, that is to say, fifteen inches above the cistern, enclosed within the wooden case of the barometer, nearly in contact with its tube, and with a stem so long as to be read off at the upper level.* To ensure a fair average and steady tempera

In some of the best modern barometers the attached thermometer is blown in the same piece with the tube of the barometer, and this

ture, it were well to enclose the whole instrument, thermometer and all, in an outer case of leather, over a wrapper of flannel, leaving only the setting and reading parts above and below accessible, and that no more than is absolutely necessary.*

In choosing a barometer, select one in preference in which the lower level (of the mercury in the cistern) is adjustable to contact with a steel or ivory fiducial point, and that not by altering the height of the mercurial surface, but by depressing the steel point carrying down with it the whole divided scale, the zero point of which is of course the apex of the point itself. There should be a provision for clamping the scale in this position, to secure it from change while setting the upper index. Care should be taken that air have free but safe access to the lower surface.

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In transporting a compared barometer to its place of destination great care is necessary. Carry it upright, or considerably inclined, and inverted; and over all rough roads, in the hand, to break the shocks it would otherwise receive. A portable barometer" strapped obliquely across the shoulders of a horseman travels securely and well; and with common care in this mode of transport its zero runs no risk of change. If merely fastened to any kind of carriage, and abandoned to its fate, it is almost sure to be broken.

To make and reduce an Observation of the Barometer.—First read off and write down the reading of the attached thermometer. Then give a few gentle taps on the instru ment to free the mercury from adhesion to the glass,

affords, as I am informed by Mr. Glaisher, the best possible means of ensuring an exact knowledge of the temperature of the column of mercury.-R. M.

* For a permanently suspended or fixed barometer, the best thermometer would be one with a tubular bulb of equal bore and thickness of glass with the barometer tube, and extending in length from the cistern to the exposed face of the instrument, and as close to the barometric column as is consistent with the structure of the upper works. Immersion of the ball of the attached thermometer in the cistern is the worst arrangement of any.

avoiding to give it any violent oscillation. Adjust the lower level to the fiducial point, and clamp the scale, if such be the construction of the instrument. Then set the index to the upper surface of the mercurial column, placing the eye so as to bring its back and front lower edges to coincidence, and to form a tangent to the convexity of the quicksilver. If the instrument have no tubular or doubleedged index, the eye must be carefully placed at the level of the upper surface to destroy parallax. Whatever mode of reading is adopted should be always adhered to. A magnifier should be used to make the contact and to read the vernier, and the reading immediately written down and carefully entered on the register.

As soon after the observations have been made as circumstances will permit, the reading of the barometer should be corrected for the relation existing between the capacities of the tube and cistern (if its construction be such as to require that correction), and for the capillary action of the tube; and then reduced to the standard temperature of 32° Fahr., and to the sea-level, if on shipboard. For the first correction the neutral point should be marked upon each instrument. It is that particular height which, in its construction, has been actually measured from the surface of the mercury in the cistern, and indicated by the scale. In general the mercury will stand either above or below the neutral point; if above, a portion of the mercury must have left the cistern, and consequently must have lowered the surface in the cistern: in this case the altitude as measured by the scale will be too short-vice versa, if below. The relation of the capacities of the tube and cistern should be experimentally ascertained, and marked upon the instrument by the maker. Suppose the capacity to be, marked thus on the instrument, "Capacity:" this indicates that, for every inch of variation of the mercury in the tube, that in the cistern will vary contrariwiseth of an inch. When the mercury in the tube is above the neutral point, the difference between it and the neutral point is to be reduced in the proportion expressed

by the "capacity" (in the case supposed, divided by 50), and the quotient added to the observed height; if below, subtracted from it. In barometers furnished with a fiducial point for adjusting the lower level, this correction is superfluous, and must not be applied.

The second correction required is for the capillary action of the tube, the effect of which is always to depress the mercury in the tube by a certain quantity inversely proportioned to the diameter of the tube. This quantity should be experimentally determined during the construction of the instrument, and its amount marked upon it by the maker, and is always to be added to the height of the mercurial column, previously corrected as before. For the convenience of those who may have barometers the capillary action of which has not been determined, a table of corrections for tubes of different diameters is placed in the Appendix, Table I.

The next correction, and in some respects the most important of all, is that due to the temperature of the mercury in the barometer-tube at the time of observation, and to the expansion of the scale. Table II. of the Appendix gives for every degree of the thermometer and every halfinch of the barometer, the proper quantity to be added or subtracted for the reduction of the observed height to the standard temperature of the mercury at 32° Fahr.

After these the index correction should be applied. This is the amount of difference between the particular instrument and the readings of the Royal Society's flintglass barometer when properly corrected, and is generally known as the zero. It is impossible to pay too much attention to the determination of this point. For this purpose, when practicable, the instrument should be immediately compared with the Royal Society's standard, and the difference of the readings of both instruments, when corrected as above, carefully noted and preserved. Where, however, this is impracticable, the comparison should be effected by means either of some other standard previously so compared, or of an intermediate portable barometer, the

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