« ForrigeFortsett »
light polarised by reflection at considerable distances from the antisolar point. The theory of atmospheric polarisation proposed by this distinguished philosopher is, that the three neutral points in the atmosphere, and the partial polarisation of the light which it reflects, are produced by the opposite action of lights polarised by reflection and refraction which have nearly the same relative intensity.*
641. Meteors and Shooting Stars can scarcely with propriety be included among meteorological phenomena, since the whole of the facts known regarding them lead to their being classed with astronomical bodies. Of late years they have been carefully observed and their movements made the subject of scientific investigation; and attention has been forcibly drawn to their consideration by the magnificent star-shower of the night of the 13th and 14th November 1866, which had been predicted beforehand. During a shower of meteors, if the lines of the tracks of the different meteors be projected on the sky, it is found that they meet nearly all in one point. This is the vanishing point in the perspective on which their tracks are projected, and points to that region in space over which the meteors are distributed. A number of these vanishing points have been determined. Olmstead first drew attention to this inquiry regarding the distribution in space of the meteoric matter, and showed that the great star-shower of November 1833 had its vanishing point in the constellation Leo. Professor Newton, America, has collected the records of the great November showers from the tenth century, from which there is evidence of eleven instances of their occurrence; and these records prove that they recur at regu
* For an exposition of this branch of meteorology, see Sir David Brewster's papers in Dr Keith Johnston's 'Physical Atlas,' in the 'Transactions of the Royal Society of Edinburgh,' and in the ‘Philosophical Magazine,' from which the above notice of the subject has been prepared.
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. 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,