Hence, instead of the highest temperatures occurring after new moon, and the lowest after full moon, as at Greenwich and Oxford, all the highest occurred at St Petersburg after full moon, and all the lowest after new moon. This result confirms what has been stated, and proves that the moon has no immediate sensible effect on the temperature of the air near the surface of the earth. Whether it has any disturbing influence on the barometric pressure, and thence on the winds and the temperature, has not yet been even attempted to be proved. 653. The next class of prognostics are of a more certain character, being taken from those indications or appearances which experience has shown to be the precursors of changes of weather; or rather, they are the first indications or beginnings of these changes. Perhaps the very earliest indication of a change from dry to wet weather might be obtained from observations of the polarisation of the atmosphere, from the circumstance that the intensity of the polarisation is reduced. by the crystals of ice floating in the atmosphere, being deposited there by the moist current which has already begun to prevail at these heights. This prognostic can be less frequently applied in such damp climates as that of Great Britain than in Africa, Hindostan, and other regions characterised by dry and rainy seasons. It is probable that when more is known of this peculiarity of the atmosphere, especially as modified by the presence of minute, invisible ice-crystals at great heights, the approach of the rains, on which so much depends, may be predicted some days earlier than can be done at present. 654. The visible form of these crystals, known as the cirrus cloud, has already been referred to, as indicating the approach of storms. But there is one form of it which may be here adverted to, from its connection with the great atmospheric currents of the northern hemisphere. It is thus given in Dr Arthur Mitchell's 'Prognostics: "The farmers in Berwickshire say that a long strip of cloud, sometimes called by them a salmon, sometimes called Noah's ark, when it stretches through the atmosphere in an east-and-west direction, is a sign of stormy weather; but when it stretches in a north-and-south direction, is the sign of dry weather." is also current as a weather-saw in the following distich: "North and south, the sign o' drouth; East and wast, the sign o' a blast." It has been shown in par. 479, from William Stevenson's observations of the motion of the cirrus cloud, and from the isobarometric lines (Plates I., II., and III.), that the upper atmospheric current in great Britain is generally from N.W. to S.E. when the barometric pressure in Europe is about the average. Hence, when the cirrus cloud-the salmon or boat form being merely the effect of perspective—stretches from north to south, or, more accurately, from N.W. to S.E., atmospheric pressure is at the time, at least in regions immediately surrounding Great Britain, at the normal height; and there being thus no disturbance indicated, "drouth," drought, or settled weather may be looked for. But when the cirrus stretches in bands lying east and west, or from S.W. to N.E., there is great atmospheric disturbance indicated, pointing to a system of low pressures (the characteristics of a storm) somewhere to the west or south-west, from which a column of moist warm air is ascending and flowing as an upper current over Great Britain; and from experience we know that this current will ultimately prevail lower down, saturating the air as it descends, in preparation for the storm which is advancing. Hence, when the cirrus lies from west to east, a storm is imminent. 655. In addition to the prognostics from clouds already stated in the chapter on CLOUDS, another may be referred to here-viz., "the pocky cloud," which was first described by the Rev. Dr Charles Clouston, Sandwick, Orkney, and stated to be followed invariably by a storm. in about twenty-four hours. This accurate observer com pares it to "a series of dark cumulus-looking clouds, like festoons of dark drapery, over a considerable portion of the sky, with the lower edge well defined, as if each festoon or pock was filled with something heavy; and generally one series of festoons lies over another, so that the light spaces between resemble an Alpine chain of white-peaked mountains. It is essential that the lower edge be well defined, for a somewhat similar cloud, with the lower edge of the festoons fringed or shaded away, is sometimes seen; it is followed by rain only."* This cloud is probably caused by large volumes of saturated air forcing their way through drier and colder air, the form of this cloud suggesting moist air diffusing itself horizontally, or from above, just as the formation of the cumulus cloud, or the steam from a steam-engine, indicates diffusion upwards. If this supposition be correct, it shows the moist, warm current to be in greater strength than usual, and a sudden commingling of air-currents, differing widely in temperature and degree of saturation,—the very conditions of a storm. This cloud is well known, and is much dreaded by Orkney sailors. Since Dr Clouston has drawn attention to this cloud it has been several times observed farther south. In the autumn of 1866 the Hon. Ralph Abercrombie observed it twice in the west of Scotland, when it was on each occasion followed by a storm. It was also observed by R. Ballingall, at Eallabus, Islay, on the day before the great hurricane which swept over Scotland on the 24th January 1868. 656. The south-east wind, when rightly understood, is a most valuable weather-prognostic in Great Britain. There are two totally distinct kinds of south-east wind,-the one presaging stormy weather, and the other settled fine weather. The characteristics of the former are moisture and warmth, and the sky clear down to the horizon, or streaked with cirrus clouds, and then covered with clouds bringing rain. The wind itself is caused by an in-draught towards the low baro * 'Explanation of Weather-Prognostics of Scotland,' by the Rev. Charles Clouston 1867. meter which accompanies a storm approaching from the west, and it gradually veers to the south and west as the storm advances. The other south-east wind is accompanied with dry weather, a clear sky with haze near the horizon, and a high barometer still rising or stationary. It is the result of a higher atmospheric pressure descending over Russia and central Europe, and it does not veer to the south and west. Since this, our only really settled fine weather, is almost always a consequence of the polar current coming from the north of Russia and spreading itself over Europe, it will scarcely be necessary to point out the great value of daily telegrams of the barometer and winds from Archangel and Riga in giving the first intimation of its probable approach to spread sheets of ice over our ponds and lakes in winter, to dry the land in spring for the reception of seed, and to bring sunshine in summer and autumn for the ripening and ingathering of the fruits of the earth. 657. Quite analogous to the above is the north-east wind in its relation to weather-changes. When the centre of a storm passes to the eastward at some distance south of any place, the wind which points to its approach is not a S.E. but a N.E. wind. This is particularly the case when the pressure has been for some time high in the north, which in winter occurs with protracted frosts. Hence one of the surest signs of the breaking up of a severe frost is the setting in of the N.E. wind, if it be accompanied with a falling barometer, and a green or yellowish-green tinted sky; and it is all the more certain if the sky becomes gradually overcast, and the wind veers from N.E. to N. and N.W. It is accompanied with a fall of snow, which passes into sleet as the wind shifts into the west, and finally into rain. On the other hand, if the north-east wind be accompanied with a clear sky with haze near the horizon, and a high barometer rising or stationary, and if the wind does not increase in strength, and tends to veer rather in the direction of E. and S.E., the weather will remain settled for some time-there being nothing as yet to indicate any change. 658. It is thus by a careful observing and recording of the lesser changes of the weather from day to day that the approach of the greater changes included under storms of wind, rain, or snow, may to some extent be known beforehand. It is not by a single observation made at one time that an isolated observer can best draw conclusions regarding the weather likely to happen, but from the character of the changes which have been silently going forward for some time. Hence it is desirable to set down, in a pictorial form, the different observations as they are made, so that the eye may take in at once the whole of the changes which are going forward; and thus the observer may with less trouble and with greater certainty draw his conclusions. The Rev. R. Tyas's handy little annual, How to use the Barometer,' is calculated to be popularly useful for recording, pictorially and in tables, the successive changes of the weather from day to day. STORM-WARNINGS. 659. It is in tropical and sub-tropical countries that an isolated observer may, with the greatest degree of certainty, predict the approach of gales and hurricanes. In these regions barometric pressure and the other meteorological elements are so constant from day to day, that any deviation, even a little, from the average of the hour and season in respect of pressure, the direction and strength of the wind, and the direction and amount of cloud, supposes the existence of a storm as the disturbing agent at no great distance. This important practical problem has been worked out with great success by Mr Meldrum at Mauritius. A few words will explain the method of proceeding. 660. At Port Louis, Mauritius, in July, the mean pressure at 32° and sea-level is 30.274 inches at 9 A.M., from which it falls to 30.186 inches at 3 P.M., it then rises to 30.236 inches at 10 P.M., and again falls to 30.192 at 4 A.M. The Y |