the maxima are too high."* The minima are lower than at Malvern; consequently the situation of the gardens must be favourable to terrestrial radiation. The mean minimum, also, is considerably lower than the mean minimum at Malvern. sues. In showery weather, when the clouds spread themselves out in thin broad white sheets, a decline of temperature almost always enThis spreading out of clouds is frequently seen during, or just after, thunder-storms; the massive-looking arched pillars of vapour, which indicate the tension of electricity, always lose their figure and spread over a larger space as the electric accumulation is expended during the storm. TABLE II.—Mean height of the Barometer at Malvern and London, in 1834, 1835, and 1836. The result of these observations of the barometer has tended to confirm the opinion that the movements of accurate barometers within a moderate distance (100 miles) of each other, are nearly simultaneous and equal, except when the mercury is rapidly rising and falling, then some hours occasionally intervene in the progress of the atmospheric oscillations. Still the annual mean difference between two perfect instruments within the distance mentioned, after the necessary corrections, will be a tolerably true indication of the elevation of the one above the other. By referring to the table it will be seen that the mean difference of two years between the barometer in Malvern and that of the Royal Society, in London, is .573, or something more than half an inch; which would give the elevation of the village of Great Malvern within a very few feet of the height deduced by the barometrical measurement of * Vide Philosophical Magazine, vol. 7, p. 355. the Worcestershire Beacon, published by Mr. Addison in a former volume of The Analyst. If the barometer falls to a very low point, and the wind is increasing in force, it usually blows very strong as the mercury begins to ascend. When westerly winds prevail, if the current shifts only a few points to the northward the barometer rises. We have yet to find the invariable conditions, if any such exist, which determine the changes of the weather: they are not discoverable either in the pressure, temperature, or hygrometric state of the atmosphere. It frequently happens with the barometer low, and the temperature of the air at the dew point, that dense, low, dark clouds roll over for a day together, without rain; whereas, with the barometer at the same point, perhaps higher, and under the same circumstances of temperature and vapour, rain falls from every passing cloud. Electrical and other changes which we cannot detect, no doubt occur in the higher regions, to bring about these various effects; hence a decline of the barometer-even with a temperature governed by vapour, i. e., with the dew point not lower than the temperature of the air-does not always portend rain. During winter, when the barometer rises, the thermometer usually falls; but in summer they generally rise and fall together. The reason for this will be understood from what has been said before; the rising of the barometer being generally accompanied by clear weather. The clear days of summer are warmer than the cloudy ones; but in winter the cloudy and wet days are the warmest. TABLE III.-Mean Dew Point at Malvern and London, This table is constructed from the indications of Daniell's hygrometer; an instrument consisting of a glass tube about six or eight inches long, bent twice at right angles, and terminated, at each extremity, in a bulb. One of the bulbs, which is usually coloured, contains a very delicate little thermometer and a small quantity of ether; the thermometer dips into the ether, which may be driven, by the heat of the hand, into either of the bulbs. Upon cooling the empty one, which may be done by pouring a few drops of ether upon it, the other immediately becomes cooled also, from the evaporation of the fluid inclosed within it, while the thermometer, dipping into it, shews how much it is cooled. When using the instrument, you narrowly watch the coloured bulb while the cooling process is going on; and at the moment when it becomes wetted with dew, you note the degree at which the thermometer included in it stands, and this is the dew point. Sometimes a few drops of ether are sufficient to produce the effect, and the inclosed thermometer falls perhaps only a degree or two; this shews that the dew point is hardly below the temperature of the air, and the atmosphere is damp. At other times it is required to wet the bulb several times; and dew is not produced upon the coloured ball until the inclosed thermometer falls ten, fifteen, or twenty degrees below the temperature of the air; the atmosphere, under these circumstances, being very dry. This instrument is usually fitted to a little brass pillar, having affixed to it another small thermometer, which shews the temperature of the air; so that the comparison between this and the dew point may be made at the same moment. That the object and use of the hygrometer may be thoroughly understood, it should be remembered that the atmosphere consists of two essentially distinct fluids, one consisting of permanently elastic gases-constituting the air, properly so called-the other of aqueous vapour, which, within the range of the temperature of the atmosphere, is capable of assuming the æriform, the fluid, and the solid state. It is upon this that all the most important meteorological phenomena depend, such as dew, fog, cloud, rain, hail, or snow. It is to discover the existing quantity of this aqueous vapour when in its æriform or invisible shape, relatively to the temperature of the air, that Daniell's hygrometer is made use of. The dew point frequently is as high as the temperature of the air, during heavy rain, in damp weather, in the evening, and at night. The dew point is very often much below the temperature of the air in clear fine weather, and especially with N. E. winds; but it can never be higher than the temperature of the air: when the latter falls (supposing them at the same point) the former must fall also the superfluous moisture (i. e., some portion of the invisible vapour) being condensed either into dew, fog, clouds, or perhaps rain. In autumn, when the temperature of the air is advancing during the day, it often happens that the dew point advances as much, and in the evening, when the air is again cooling, the dew point must fall; its subsidence being accompanied by a copious deposition of dew, and where the lower strata of the atmosphere are chilled by radiation, by the appearance of mist or fog. The form in which moisture under these circumstances is deposited upon the ground, is much modified by the state of the air: if the wind blows strongly, surfaces become moist and perhaps wet; it is only when the air is calm, that those minute drops, standing at the very tips of the blades of grass and upon every, the minutest fibre, constituting dew properly so called, can be seen. Air of the same temperature affects our sensations differently; the impression is greatly modified by the force of the wind and the state of the dew point. When the atmosphere is calm, the temperature moderate, and the dew point very high, it seems close, warm, and oppressive-sensations much diminished by a light breeze; on the other hand, when the temperature is moderate, the air calm, and the dew point very low, the feeling is cool, bracing, and pleasant; but if the wind blows fresh, we then feel it cold, harsh, and disagreeable; this last condition is very apt to induce catarrh, sorethroat, and rheumatism in those disposed to these affections. It is when the dew point is high that ladies' hair falls out of curl, a ringlet is an elegant and delicate hygroscope. Every one must have observed, occasionally, the visible condensation of the breath; this is a tolerable indication of the state of the vapour of the atmosphere, such an appearance shewing that the temperature of the air and dew point are quite, or very nearly, the same: it may sometimes be remarked in the open air, before, almost always during, or just after, continued heavy rain. During the winter season, (and the remark will apply generally to the autumn and spring), a rise in the thermometer and dew point at the same time, is a sure indication of clouds and rain. It often happens on a clear sunny day after rain, when evaporation is going on with the utmost rapidity, that the dew point does not rise, or, in other words, that the quantity of vapour in the lower regions of the atmosphere does not increase, nor do clouds form in the higher; the vapour, therefore, must be drawn off to distant regions. Sometimes when the weather is very foggy in the morning, the hygrometer exposed to it will not be dewed or moistened until the temperature of the dark bulb be reduced two, three, or four degrees. When this occurs, the fog is usually succeeded by a clear and fine day. The following Table of the wind is arranged in accordance with the remarks made in the second volume of The Analyst, p. 221; and it appears that those to the south of the east and west points of the compass, in comparison with those to the north of these points, were, in the former year, as 15 to 10 at Malvern, and in the latter as 18 to 10; and they bore very nearly the same relation to each other in London-the numbers being, for the first year, 15 and a fraction to 10, in the last 19 to 10. TABLE IV. Of the Wind at Malvern and London for the Seasons of 1834-5 and 1835-6. Two or more different currents of wind may be frequently detected by observing the clouds. Sometimes the higher ones may be seen nearly stationary, the lower moving rapidly, either at various angles to the slow-moving higher ones, or now and then in an opposite direction. The primary indication of a change in the direction of the wind may be detected sometimes by noticing the appearance of clouds, even before they are influenced in their movements by the ærial current. They display a great variety of circular segments or curls, in a plane apparently parallel to the horizon the convex face of the curl when below the zenith being turned downwards towards the horizon, whereas the convex arch of the cumulus is turned upward towards the zenith. The appearance here spoken of may be difficult to describe, but it is very character istic. The convexity of the curl is generally in the direction of the approaching current. When the air is calm, evaporation is proportionate to the interval between the dew point and the temperature of the air; it is ac |