lar intervals of thirty-three years and part of a day, the date of their occurrence having been from the 9th to the 14th November. In addition to the annual period in November, meteors also are frequently observed about the 11th of August; and, though not so general, at other times of the year. These showers indicate the regions of space through which immense numbers of meteors are moving with planetary velocity; and when the earth in its motion round the sun travels through these spaces, it attracts to it many of these cosmical bodies, which, as they enter the atmosphere with a speed of about thirty miles a second, are set on fire by the rapidity of their flight, like the arrow shot by Acestes

"Chafed by the speed, it fired; and as it flew,

A trail of following flames ascending drew." Meteors move in orbits variously inclined to that of the earth. Alexander Herschel has particularly studied this class of bodies, and, from twenty well-observed cases, has determined the weight of the meteors as varying from 30 grains to upwards of 7 lb.

642. Since they become luminous on entering the atmosphere, it is evident that careful observations made at different places would give an approximate solution to the problem of the height of the atmosphere. From such observations it is concluded that the earth's atmosphere is at least one hundred miles high, and that probably, though in an extremely attenuated form, it reaches to about two hundred miles. The extraordinary quantity of latent heat in air so attenuated, becomes sensible as the air is compressed before the meteor, and thus we have a satisfactory explanation of the speedy ignition of all meteors as they traverse the atmosphere, and the rapid conversion of the smaller ones into thin mist, which in some cases remains floating in the sky for half an hour. Meteors enter the atmosphere bringing with them the temperature of the stellar spaces, which Sir John Herschel supposes to be about -239°; and since their surfaces quickly become highly heated, an explosion soon takes place from

the difference of the temperatures. Such explosions, however, do not always take place; thus the meteoric mass which fell at Dhurmsala, in India, on the 14th of July 1861, was first very hot, but as the great cold of the interior of the meteor quickly counteracted the heat produced on its surface by its rapid flight through the air, it soon became so intensely cold that it could not be touched.

643. Speculation has gravely attempted to bring this interesting class of bodies within the legitimate domain of meteorology, by supposing that a shadow from the annular ring of meteors rotating round the sun falls on the earth at certain seasons-viz., in February, May, August, and November-in consequence of which part of the sun's heat is cut off, and the temperature of the earth therefore falls. It is conclusive proof against this theory, that when these interruptions of temperature are particularly investigated, it is found that the date of their occurrence varies backward and forward to some extent from year to year; that the duration of their occurrence varies from three to six days, or sometimes longer; that they are accompanied with a prevalence of the polar current, as indicated by northerly winds and increasing pressure in advancing northward; and that instead of appearing simultaneously at different places, they are propagated from place to place like other changes of temperature, thus pointing to a terrestrial instead of an astronomical origin.

V

CHAPTER XIV.

WEATHER, AND STORM-WARNINGS.

644. WEATHER is the condition of the air at any time as regards heat, moisture, wind, rain, cloud, and electricity; and a change of weather implies a change in one or more of these atmospheric elements. From the direct bearing which weather-changes have on human interests and pursuits, they have been closely observed from the earliest times, in order that their approach might be predicted with some degree of confidence. The strong craving in the public mind. for this knowledge is attested by the prognostics current in every language, which, amid much that is shrewd and of practical value, embody more that is vague, and not a little. that is absurd. Any reference to Moore and other almanacmakers is unnecessary, except as testimonies to a widespread ignorance of even the most palpable elements of physical law, which is a disgrace to the educational system of the country. When prognosticators of higher pretensions appear before the public with revelations, weeks or months beforehand, of fine or stormy weather fraught with great advantage or incalculable disaster to agricultural and other interests, it is curious to note how their predictions are laid hold of by the newspapers and scattered broadcast over the country.

645. The truth is, no prediction of the weather can be made, at least in the British Islands, for more than three, or perhaps only two days beforehand; and any attempt at a longer prediction is illusory. But though no prediction of

the weather weeks or months beforehand can be made with any pretension to trustworthiness, yet guesses or surmises may be formed which are not without value. All prediction based on solar or other astronomical causes, if not misleading, is useless. Investigations appear at present to point to a connection between the positions of the planets on the one hand, and the sun's spots, terrestrial magnetism, and the aurora, on the other hand. Nothing, however, could be inferred from such a connection, even were it conclusively established, that could be turned to account in predicting the weather likely to occur in a particular country within a specified time. The only safe guides we can have in attempting to forecast the weather for some time are averages based on terrestrial observations.

646. Of this class may be mentioned the interruptions which occur in the regular march of temperature in the course of the year, of which some account has been already given at page 140. Thus, since in Scotland at least, cold weather prevails some time in the second week of February, April, May, August, and November, and in the end of June; and warm weather in the second week of July and August, and in the beginning of December, it follows that, when at these times the weather begins to grow cold or warm, a continuance of such weather may be expected for a few days. Since these interruptions of temperature arise from a different distribution of atmospheric pressure from what usually prevails, they are generally either preceded or followed by stormy weather.

647. If, after an unusual prevalence of south-west wind, or the equatorial current, the polar current or north-east wind should set in, it is probable that easterly winds will prevail for some time. If the season be winter, frost, and perhaps snow, may be looked for; and if summer, the weather will become dry, warm, and bracing, particularly if the wind be E. or S.E. But suppose easterly winds have largely predominated in autumn, and south-westerly winds begin to prevail in the end of November or beginning of December,

the weather is likely to continue exceptionally mild, with frequent storms of wind and rain, till about Christmas. This period occurs nearly every year, and its beginning is popularly known as St Martin's summer. On the same principle, if easterly winds preponderate largely above the average in spring, the summer is likely to be characterised by southwesterly winds, with much rain and moisture, and little. sunshine; but if easterly winds nearly fail in spring, they are likely to prevail in summer, bringing in their train dry, warm, bracing weather, clear skies, and brilliant sunshine. This latter prognostic is confidently believed in by several meteorologists; but a comparison of the weather of spring with that of the succeeding summer during the last eleven years in Scotland shows, that while it holds good in the majority of cases, the number of times it fails are too many to entitle it to the claim of a trustworthy prognostic.

648. It is an opinion which has been long and popularly entertained that the changes of the moon have so great an influence on the weather that they may be employed with considerable confidence in prediction. That the moon's changes exercise an influence so strongly marked as to make itself almost immediately felt in bringing about fair or rainy, or settled or stormy, weather, an examination of meteorological records, extending over many years, conclusively disproves.

649. Sir John Herschel states, in his Familiar Lectures,' that the moon has a tendency" to clear the sky of cloud, and to produce, not only a serene but a calm night, when so near the full as to appear round to the eye." Arago says, "La lune mange les nuages." If these opinions are founded on fact, then the moon must have a very great and decided influence in clearing the sky of clouds. William Ellis has examined the Greenwich meteorological records from 1841 to 1847, and shown from these seven years' observations that such a peculiar and striking effect does not exist.* The popular opinion probably arises from the circumstance that the * Philosophical Magazine' for July 1868.