direction of the wind after the complete passage of the hurricane is a point of interest, as indicating whether the "cyclone" consist in the bodily transfer of a given mass of rotating air, or in the successive transmission of a rotary movement from air to air in situ, the air in each point of its track being only transiently agitated.

Winds. The points most important to remark respecting the wind are—

1st. Its average intensity and general direction during the several portions of the day devoted to observation.

2ndly. The hours of the day or night when it commences to blow after a calm, or subsides into one after a breeze. 3rdly. The hours at which any remarkable changes of its direction take place.

4thly. The course which it takes in veering, and the quarter in which it ultimately settles.

5thly. The usual course of periodical winds, or such as remarkably prevail during certain seasons, with the law of their diurnal progress, both as to direction and intensity— at what hours and by what degrees they commence, attain their maximum, and subside-and through what points of the compass they run in so doing.

6thly. The existence of crossing currents at different heights in the atmosphere, as indicated by the courses of the clouds in different strata.

7thly. The times of setting-in of remarkably hot or cold winds, the quarters from which they come, and their courses, as connected with the progressive changes in their temperature.

8thly. The connexion of rainy, cloudy, or fair weather with the quarter from which the wind blows, or has blown for some time previously.

Several of these points of inquiry have especial reference to land winds, and can only be duly studied in port or during residences on shore. In cruises along shore, or on arrival or departure, observe how far the influence of the land extends, and by what gradations the character of the

winds changes from terrestrial to oceanic, especially with reference to the difference between the hours when the sun is above and when below the horizon.

Clouds and Fogs.-The dissipation of cloud under the apparent influence of the full moon is a point to which attention has lately been called: the state of the sky on the days and nights of the full moon, and those immediately preceding and following it, should be noted with a view to confirming or refuting this connexion. Hourly observations, commencing before sunset, of the relative proportion of clear and clouded sky would be desirable.

The height of the vapour plane is a datum of importance, especially in tropical regions. At sea it is not easy to determine; but when near a mountainous coast, where the clouds repose at definite levels on the hills, many opportunities may occur of ascertaining it with precision. The lower level of unmelted snow may in such localities also be made a subject of inquiry. The average height of the vapour plane at sea under the equator is a meteorological element of much interest. Opportunities of determining it by measuring the apparent altitude of the flat bases of cumular clouds from two ships whose distance is known, or otherwise, should be seized when they offer.

When the sky suddenly clouds over, or when fogs form unexpectedly, the barometer and thermometer should be noted minutely, as such appearances often result from a rapid diminution of atmospheric pressure and consequent absorption of heat by the rarefaction of the air. Fogs prevailing at definite localities indicate a temperature of the sea habitually exceeding that of the air. They often also indicate the vicinity of ice. Whenever fogs are met with at sea, the temperature of the air and of the surface-water should be recorded with more than usual care.

Temperature of the Sea.-Shoals cast up water from a lower level to the surface where any current exists, and therefore a sudden change of temperature of the surface may indicate a shoal. In crossing currents coming from warmer or colder latitudes, the surface temperature should

be especially attended to, and the maximum of irregularity due to the current watched for. Should opportunities offer of obtaining deep-sea temperatures, they should be eagerly seized.

Blue Colour of the Sea.-The cause of the blue colour of the sea and of deep or still lakes is still very imperfectly understood, and opportunities should be taken in calm weather to make observations which may tend to explain it. This may be done by an exploring-tube of the following simple character:

A A is the water surface. B the tube. CC a glass to insure a perpendicular incidence of the visual ray. D, D, D, D, small air-holes to let air escape from the open subaqueous part of the tube F F. EE a diaphragm to cut off sidereflected light.

This is to be directed in all azimuths and inclinations to the horizon, in as still water as may be had, if only very deep and blue. The points to be observed are:—

1. The colour of the emergent light in all azimuths and inclinations.

2. Polarization, if any, when the water is very calm and the sun strong.

3. The possibility of directing a telescope down such a tube to examine the bottom.

4. The possibility of forming CC into an object-lens and converting the whole apparatus into a submarine telescope.

It is probable that the blue colour of the sky and of deep still water is referable to the same cause, and any observations tending to prove or disprove this would be valuable.

Observations in Port or in Temporary Residences on Shore.— Opportunities should not be lost of ascending lofty eminences, and noting thereon the hygrometric and thermometric conditions corresponding to altitudes measured by the portable barometer, or otherwise known.

The temperature of deep wells should be ascertained and that of the soil at different depths, which, if made with due care and under favourable circumstances, is an observation of very great interest. Excavations should be made in dry soil, and under fair exposure to sun and wind, in which should be buried, at depths of three, six, and nine feet, thermometers well wrapped in woollen cloth, or in pots of pounded charcoal, or even of dry sand, enclosed in strong vessels to defend them from damage, and to prevent the possibility of change of temperature in extracting and reading them. The zero points of the thermometers should be most scrupulously ascertained, and their errors at the temperatures registered made a subject of special inquiry. The readings should be exact to tenths of degrees. Observations thus made under the equator in various longitudes, with scientific precision, might furnish data of the utmost value towards determining the constancy or variability of the sun's radiation from year to year. If the thermometers cannot be spared, bottles of water similarly defended may be buried. (or of brandy, if in frozen soil), and the temperature of the liquid taken immediately on raising them. In case of prolonged sojourn this should be repeated monthly.

Some localities are remarkable for enormous falls of rain. Thus it is stated, on the authority of Captain Roussin, that between the 1st and 24th of February, 1820, there fell

twelve feet seven inches of rain in the Isle of Cayenne.* In all such localities great attention should be paid to the rain-gauge, and pains taken to procure extracts from perfectly authentic registers containing instances of the kind, and information respecting their attendant circumstances. In some geographical localities it is said never to cease raining-in others, that rain never falls. Local inquiry and consultation of records must here stand in lieu of personal observation, due care being taken to rely on none but unexceptionable evidence.

The phenomena of dew are of more interest, and can be better studied, on shore than at sea. The amount of dew collected by a given surface of any bibulous radiant, as cotton, &c., in clear nights, in exposures perfectly open to the sky, and on the level of the soil, should be registered. If accompanied with observations of the depression of the terrestrial radiant thermometer, and also of the hygrometer, such observations would acquire additional value.

The temperature of the soil under the direct influence of the sun, as indicated by a thermometer barely covered with dry earth, is an element of importance to the botanist, and may be recommended as an apt accompaniment to actinometric observations. The thermometer used should have a scale reading at least to 180° Fahr.

Meteorological registers, kept by persons of credit at places where the ship may remain or touch, should be inquired for and copied, or the originals procured; and the instruments with which the observations have been made should be carefully compared, and the height of their stations above the sea-level ascertained.

Waterspouts, Bull's-eye Squalls, Whirlwinds.-The transition from the mere eddy to the whirlwind, and from the whirlwind to the waterspout (Trombe), should be traced if pos sible. All circumstances from the first trace or prognostic to their final dissipation should be minutely noticed, espeEdinb. Phil. Journ., viii. 186, from Silliman's American Journal of Science, iv. p. 375.