with water to 0° of the scale. Sometimes both the tubes are graduated, in which case, the amount of the depression of the fluid in one branch of the guage, and of its ascension in the other, being added, gives the force of the wind. If only one of the branches be graduated, then twice the amount of the ascent or descent from 0 of the scale is the height of the column of water supported.

The length of the column being observed, the following table (extracted from the Report of the Committee of Physics of the Royal Society) will give the force of the wind on every square foot of a body opposed to its direct action:—

In great degrees of cold, a saturated solution of sea-salt may be used instead of water, the specific gravity of which is 1.244. If the force in the above Table, for any height, be multiplied by the specific gravity, the product will be the true force, as measured by the solution.

If an index be added to the instrument, and the disc on which it stands be divided like a compass card, then, by setting the instrument with the North and South points of the card in the direction of the meridian (true); or if magnectic to be so specified, the direction, as well as the force of the wind, will be indicated.

DROSOMETER.

This is an instrument for measuring the quantity of dew. We recommended for this purpose a metal funnel, of five inches in diameter, sloping at an angle of 30°, and terminating in a small hole and very short pipe. This funnel must screw into the neck of a glass tube of equal bore, and a quarter of an inch in diameter. This tube must be graduated, so that each degree may indicate the thousandth part of an inch on the external aperture. The bottom of the tube must screw into a solid foot, so that the instrument may stand firm. To observe with this instrument, see OPERATIONS.

ELECTROMETER.

There are few meteorological data more interesting than those respecting the electrical state of the atmosphere; but, unfortunately, this is a subject not yet fully understood. The electricity of the air is observed by means of electrometers or electroscopes, of which there are several of various constructions, as De Saussure's, Volta's, Singer's, Bohnenberger's, &c. Leslie prefers Singer's, but Bohnenberger's has the advantage of indicating the kind of electricity communicated to the conductor.

Whichever of these instruments is used, and the choice may be left to the option of individuals, it must be connected by a fine copper wire with a conductor, which as the report of the Committee of Physics of the Royal Society observes, may be any temporary contrivance as "a common fishing rod, having a glass stick well varnished with shell-lac, substituted for its smallest joint; to the end of the glass must be fixed a metallic wire, terminating in a point, and to the end of this point the copper wire must be fixed which connects the conductor with the electroscope.

CYANOMETER.

This is an instrument constructed by De Saussure for measuring the intensity of the blue colour of the sky. It consists of fifty-three slips of paper, about a quarter of an inch broad, and one inch long, stained with the successive

shades of blue, from the lightest to the darkest. These are arranged in a circle, and the colour of the heavens is compared with these, and the corresponding tint determined.

The great defect of this instrument is the difficulty of obtaining comparative instruments; and even the same Cyanometer changes its tints by exposure to air and light. Other cyanometers than that described, have been proposed, but none that we know of are perfect. This is to be regretted, as the intensity of the colour of the sky, varying with the state of the atmosphere, and with the heights at which the observation is made, it becomes interesting to note the tint of the heavens under different circumstances.

PHOTOMETER.

This instrument, though not strictly necessary to the traveller, is so extremely portable, particularly that form of it which is called the portable photometer, that we cannot but recommend to the traveller to have one with him. The object of this little delicate instrument is, to ascertain the relative intensities of light, as depending either on the times of the day, the seasons or the heights of places. Its applications are very various, and it serves to supply some very interesting data of general physics.

INSTRUMENT FOR OBSERVING THE DIRECTION OF THE

UNDULATORY MOTION OF EARTHQUAKES.

For this purpose treacle or other viscous substance has been recommended, which being put into a bason, will, by the marks it leaves after agitation, show the direction of the wave, and in some degree its force; but treacle or other viscous fluid is not always to be had, particularly by the traveller, and moreover, in order to render the observations made at different places comparative, the same substance and of the same specific gravity should be employed. Fortunately water will answer the purpose though not so well as treacle.

We recommend then, a tin or copper bason, hemispherical within, and having its inner surface whitened but not polished; the composition used must be such as water will wet but not dissolve. From a point in the exact centre of the bottom, let black lines be drawn to the number of 32, and at the top let the usual letters and degrees of the compass be marked; in addition to this let black and equidistant horizontal circles be painted on the inner surface of the basin, these may be

a quarter of an inch apart, the bason being six inches in diameter. Set the bason with its North line in the direction of the true North, then pour into it a quantity of water equal in depth to one inch, and sprinkle on its surface some light powder, no matter what, so that it will float and not dissolve, as fine sawdust, or bran, or scrapings or raspings from a bit of wood, or snuff, or any substance of like kind. The bottom of the bason should be lested with lead that it may stand firm. Any undulation of the water will leave the powder adhering to the side, and thus show at once both the direction and strength of the shock; the latter being indicated by the height of the horizontal line to which the water attains.

EUDIOMETER.

This is an instrument for estimating the quantity of oxigen contained in atmospheric air; many and various Eudiometers have been constructed, but as the experiments that have been made with them prove, that the constituent elements of the air are nearly identical in all places, at the tops of the highest mountains, and in the plains, and in all parts of the world; and as, moreover, experiments with the Eudiometer require the use of certain re-agents, it is an instrument which the traveller may very well dispense with.

INSTRUMENTS FOR OBSERVING TERRESTRIAL MAGNETISM.

These, for a traveller, will be a dipping needle furnished with intensity needles, a variation compass, and the small portable apparatus for the horizontal intensity, usually called Hansteen's apparatus. For the reasons why we merely indicate these instruments, without giving any description of them, see OPERATIONS.

The remaining instruments mentioned in our list require no particular description.

Great care must be taken in transporting some of the above described instruments, as they are delicate and subject to derangement. Until the traveller comes upon the ground of his observations, some of the more delicate of the instruments may be put among his linen, which, by its elasticity, will secure them from shocks and other accidents.

SECTION II.

OPERATIONS.

The scientific traveller, adequately supplied with proper instruments, needs no instruction from us regarding the application of them to the several operations he may have to perform in the course of his travels: but for the unscientific, and for those who cannot take instruments, or may have lost them, we shall point out various modes of operation sufficiently exact for general purposes. Many of these operations comprise what is termed Practical Geometry on the ground, on which subject a treatise was formerly published by Professor Landemann, of Woolwich, some of whose more necessary problems we shall give as eminently useful to the traveller.

Previous however to treating of operations requiring the aid of even the simplest contrivances, we will speak of

MEASUREMENTS BY THE EYE.

There are many cases, in which the traveller is desirous of ascertaining heights and distances, which he cannot measure for want of even make-shift instruments, or because he has no time for measuring, or because the objects are inaccessible. It is therefore of great consequence that he should have so practised his eye previously, as to be able to judge by it alone, with considerable accuracy, of heights and distances; and this he must learn to do under all circumstances of atmospheric influence, as objects appear nearer or further removed, more or less high, larger or smaller, according to the different states of the atmosphere. In estimating heights and distances by the eye it must be remembered, that the angle subtended diminishes with the distance, so that in measuring a height or distance by the repetition of any unit, whether it be a hundred yards or ten feet, a less and less apparent space must be taken for the unit, as the part to which it is applied recedes from the eye. Thus it will be found that if a person who can judge very accurately of the dis