the specific gravity of the water he may wish to examine by means of the hydrometer described Sect. INSTRUMENTS.

If it be required to take the specific gravity of the water of a spring, of a well, or even that of the sea, or of a lake or of a river at its surface, all that is necessary is to take up, in any convenient vessel, a quantity of the water sufficient for the immersion of the hydrometer. But if it be required to take the gravity of water drawn up from a depth, then recourse must be had to the instrument we have described for drawing up water from a depth.

Two precautions are necessary; first to filter the water, if it contain any insoluble impurities floating in it, and secondly to note the temperature of the water. This last is essential, for, before the specific gravity of the water can be exactly known, its temperature must be reduced to the temperature of maximum density. There are tables for this, but the reduction need not be made till the traveller return to his head quarters.

TO BRING UP WATER FROM CONSIDERABLE DEPTHS.

The mode of doing this is sufficiently explained by the description and use of the instrument recommended for the purpose and described Sect. INSTRUMENTS.

TO ESTIMATE THE EVAPORATION FROM STANDING OR RUNNING

WATER.

We know of no method by which this can be done directly and exactly. It can only be approximately calculated from observations with Leslie's Atmometer, or with the hygrometer, see Atmometer Sect. INSTRUMENTS.

TO OBSERVE THE QUANTITY OF RAIN, SNOW, OR HAIL.

For observations of the rain which falls see Ombrometer, Sect. INSTRUMENTS. As for the quantity of snow which falls we may remark, that if their falls but little, it is hardly worth observing, as the small quantity that could be collected would not give any satisfactory result. If however the snow melts as soon as it falls, it may be collected in the Ombrometer like rain. But when much snow falls, the

better way is perhaps to look out for a spot where it has neither been accumulated by drift nor carried away by wind, and there to take the depth at which it lies; after which take up a square foot of it as exactly as possible, thaw it and measure the cubical inches of water it furnishes. The duration of the fall should be noted, and the snow taken up as soon as the fall is over.

With regard to hail it may be collected in the rain guage, but as the hail stones are apt to rebound, the estimate will probably be short of the real amount. The box of the guage without its funnel would answer better. But by whatever means the hail stones are collected on a given space, they must be thawed and the quantity of water resulting, measured. Observe also if there be any sediment in the water resulting from the thawed hail, and if so, in what quantity and of what kind; it may be common sand or fine volcanic dust, &c.

TO OBSERVE THE QUANTITY OF DEW WHICH FALLS.

The Drosometer to be employed for this purpose, and which we have described Sect. INSTRUMENTS, should be used as follows

Blacken the inside of the tin or copper funnel over the flame of a candle or lamp, so as to cover it completely with a coating of soot. Then screw it into the graduated tube, and the tube into its stand. Set the instrument on the ground, out of the way of shade or current. In the morning observe the quantity of dew collected and note it down.

TO OBSERVE THE FORCE OF THE WIND.

For this purpose we have recommended the use of Lind's wind guage, which see, Sect. INSTRUMENTS.

To observe with this instrument it must either be held in the hand, or, still better, securely placed in some open situation, away from all eddies and reflected currents of wind. The date and hour of the observation must be recorded, the quarter whence the wind began to blow, and the direction it takes in veering. When the wind blows in gusts, the maximum effect of these should be noted.

TO OBSERVE SOLAR RADIATION.

For this purpose, the particular thermometer described in Section INSTRUMENTS should be employed. It is recommended to set it about an inch above the bare soil and screen it from currents of air. It must of course be so placed as to receive the full influence of the sun's direct rays at those hours of the day when they are hottest, and the maximum of each day should be regularly entered. The observations can be regular only when the traveller is stationary, but it is interesting to note the intensity of solar radiation, when it can be done, during even a temporary halt, noting the date and hour when the observation was taken, and whether the heat was still increasing, or was stationary or diminishing, at the time of taking up the instrument.

TO OBSERVE TERRESTRIAL RADIATION.

The observation of terrestrial radiation may be more regularly effected by the traveller than that of solar radiation; for he generally rests at night, and has therefore many opportunities of setting out the parabolic metallic mirror we have described, and noting the indication of its thermometer, in the morning before he resumes his journey. The instrument must be screened from currents of air, but away from the vicinity of any high buildings, trees, or other objects which conceal a considerable portion of the heavens. It is better to set it on the top of a house than on the ground near it; but, in all cases, its height above the ground and the nature of the soil or other object over which it is immediately placed, should be noted, and the direction in which. unavoidable obstacles to free exposure exist, with the proportionate part of the heavens they hide from the instrument.

TO OBSERVE THE ELECTRICAL STATE OF THE ATMOSPHERE.

It were very desirable that the electrical state of the atmosphere should be observed at the regular hours that other meteorological observations are made; but, although

make-shift conductors may be occasionally set up, yet the traveller cannot always command even such: besides which, he cannot always observe at stated times, nor, after all, is so capricious an element as atmospheric electricity sufficiently understood for casual observations to be of much use. If, however, a traveller choose to take with him one of the electrometers we have mentioned, Sect. INSTRUMENTS, and remains for a sufficient length of time at any place, he may set up a conductor and observe the indications of the electrometer; and this he should do, not only at the stated hours, but also before, during, and after storms, rain, snow, &c. When the electroscope is charged, unless Bohnenberger's is used, the nature of the electricity may be tested in the usual way, by excited glass or sealing wax.

TO OBSERVE TERRESTRIAL MAGNETISM.

The magnetism of the earth is now exciting that general attention among philosophers to which it is so justly entitled. It was, therefore, our intention to give the traveller some short instructions relative to such observations as he might have opportunities to make upon it with the more ordinary and portable instruments. But, reflecting on the great nicety required in making magnetic observations, in order that they may be really useful, and that, as we have before observed, a kind of apprenticeship is absolutely necessary to enable any one to observe properly, we have thought that a few words on the subject would be worse than nothing, while to enter into details would lead us too far. We have therefore merely named the instruments which a traveller may take with him without inconvenience, and with which, when he is properly instructed in the mode of observing, he may obtain much useful and interesting data. He is recommended to read with attention the articles on magnetism in the fifth and eighth parts of Taylor's Scientific Memoirs, and the remarks on instruments and methods, in the eighth volume of the Reports of the British Association.

TO COLLECT THE GASES FROM VOLCANOES, SPRINGS, &c.

This is an operation which the casual traveller can hardly have time or opportunity to perform, as it is however de

sirable, to ascertain precisely the nature of the gases given out by volcanoes, springs, &c. we will point out a mode by which this may be effected.

Fill a bottle with water and invert it into a basin having some water in it and a slip of wood laid across with a notch cut out to receive the neck of the bottle. Insert one end of a double bent tube* having this form into the nozzle of a funnel by means of a cork or otherwise, so that gas may not escape at the juncture. Slip the other end of the bent tube into the neck of the bottle. Having thus arranged your pneumatic trough, fasten all firmly, so that nothing may move, and set the apparatus with the inverted funnel over the evolving gas. If there be any inconvenience or danger attending a too near approach, fix the whole to a ring, at the end of a pole, like the ring and handle of a landing net, and hold the apparatus over the gas till the bottle be full, which it will be when all the water is expelled. Then remove the tube, and cork your bottle under water. The water used must not be thrown away, but bottled for examination, as it may have imbibed some principles from the gas which passed through it.

A portable apparatus for collecting the gas might be easily imagined, and would be found very useful on many oc

casions.

TO OBSERVE THE DIRECTION OF THE UNDULATORY MOTION OF

EARTHQUAKES.

When a traveller visits regions where earthquakes are frequent, he would do well to provide himself with such an instrument as is described in the Section INSTRUMENTS; and as no warning is given of an approaching earthquake, he should always set his instrument wherever he may halt for any time. The description of the instrument sufficiently indicates the mode of observing with it.

TO MEASURE THE HEIGHTS OF MOUNTAINS.

Trigonometrically.-The only correct method of obtaining the heights of mountains is by trigonometrical measure

* A flexible tube might, on many occasions, supply the place of a glass tube with advantage; and such a one might be made to screw on to the end of a funnel.