Mr. Daniel now kept a journal of the weather for several nionths, registering the variations of the barometer, the ther mometer, De Luc's hygrometer, and the temperature at which moisture was condensed; from which he obtained some very ⚫ interesting results,' though what they were is not mentioned. The apparatus was afterwards varied in the following manner. Five small hollow cylinders of brass were procured, three inches in diameter, and four inches in height, fitted with a small cock in the bottom of each. These were highly polished, and placed in a frame, one immediately over another, so that, by turning the cock, the contents of the upper would flow into that immediately beneath it. The cold liquid was put into the top cylinder, and when steam was produced upon its surface, the solution was suffered to run into the next, and so into the third, &c. till all condensation ceased; when the temperature of the dew point was marked as before. This apparatus, Mr. Daniell found to answer very well, the bright surface of the metal being visibly obscured by the slightest film of moisture. These experiments were, however, troublesome, and required considerable time to insure accuracy. It was not until many months after he had commenced this course of inquiry, that Mr. Daniell discovered that the mode of investigation which had been suggested to him by the remark of Pliny, was not so new as he had conceived it to be, the same principles having been ingeniously applied by the Academicians del Cimento, as well as by M. Le Roi, and Mr. Dalton, to the purposes of Hygrometry, by investigating the point at which dew is deposited.

The discovery of this want of originality damped for a time Mr. Daniell's ardour; but he remained impressed with the great utility of any contrivance that might enable an observer to mark with precision, neatness, and expedition, the constituent temperature of atmospheric vapour. On reading the account of the contrivance of Dr. Wollaston, which he has termed the Cryophorus, the subject again recurred to him, and he received from that instrument the hint which, after many trials, led to the completion of his ingenious hygrometer.

Of this instrument the following is a description. Two thin glass balls of 1 inch in diameter, are connected together by a tube, having a bore of about 4th of an inch. This tube is bent at right angles over the two balls, one arm of which contains a small thermometer, whose bulb of a lengthened form descends into the ball. This ball having been about two-thirds filled with ether, is heated over a lamp till the fluid boils, and the vapour issues from a capillary tube in which the opposite ball terminates. The vapour having expelled the air

from both balls, the capillary tube is hermetically closed by the flame of a lamp. This process is familiar to those who are accustomed to blow glass, and may be known to have succeeded after the tube has become cool, by reversing the instrument, and taking one of the balls in the hand, the heat of which will drive all the ether into the other ball, and cause it to boil rapidly. The ball from which the capillary tube issues, is now to be covered with a piece of muslin. The stand on which hangs the glass tube with the balls depending, is formed of brass; a transverse socket at its top being made to hold the glass tube in the manner of a spring, so as to allow it to turn and be taken out with little difficulty. A small thermometer is fixed on the pillar of the stand.

The mode of using the instrument, is as follows. After having driven all the ether, by the heat of the hand, into the ball furnished with the thermometer, it is to be placed at an open window, or out of doors, with the ball so situated that the surface of the liquid may be upon a level with the eye of the observer. A little ether is then to be dropped upon the covered ball. Evaporation immediately takes place, which, producing cold upon this ball, occasions a rapid and continuous condensation of the ethereal vapour in the interior of the instrument. The consequent evaporation from the included ether, produces a depression of temperature in the opposite ball, the degree of which is indicated by the included thermometer. This action is almost instantaneous, the thermometer beginning to fall in two seconds after the ether has been dropped. A depression of 30 or 40 degrees is readily produced, and the thermometer may be driven down below the zero of Fahrenheit's scale. The artificial cold thus produced, occasions a condensation of the atmospheric vapour upon the ball containing the thermometer, the first appearance of which is indicated by a thin ring of dew, coinciding with the surface of the ether. The degree at which this takes place, must be carefully noticed. When the instrument has been constructed with a transparent ball, it is recommended to have some dark object behind it, as a house or a tree; the ring of dew not being so readily perceived against the open horizon. The depression of temperature is first produced at the surface of the liquid, where evaporation takes place; and the currents which immediately ensue to effect an equilibrium, are very perceptible. The bulb of the included thermometer is not quite immersed in the ether, that the line of greatest cold may pass through it. In very damp or windy weather, the ether should be very slowly dropped upon the ball; otherwise, the descent of the thermometer will be so rapid as to render it extremely difficult to be certain of the degree. In dry weather,

on the contrary, the ball requires to be well wetted more than once, to produce the requisite degree of cold. If, at any time, there should be reason to question the correctness of an observation, the Author recommends that the temperature at which the dew upon the glass again disappears, should be noted, and the mean of the two observations (the errors of which, if any, will probably lie in opposite directions) will afford the true result. Care, of course, in every observation, must be taken, that the breath do not affect the glass.

Thus much on the construction of the hygrometer. It is simple and easy. Its graduation depends upon no arbitrary or disputed determinations of wet and dry; it is liable to no deterioration from use, age, or accidental circumstances; and, above all things, whenever or by whomsoever made, it is incapable, in proper hands, of affording erroneous results. It may be more or less boiled; the vacuum may be more or less perfect; and it may, consequently, require the affusion of a larger or smaller quantity of ether to make it act; but (provided the thermometer be correct) the observation, when obtained, cannot deceive. Its determinations are, therefore, as strictly comparable one with another, under all circumstances, as those of the barometer or the thermometer.'

p. 149. The application of this ingenious and useful instrument to the purposes of a weather-glass, are thus described.

When consulted with a view of predicting the greater or less probability of rain, or other atmospheric changes, two things are to be principally attended to-the difference between the constituent temperature of the vapour, and the temperature of the air; and the variation of the dew point. In general, the chance of rain, or other precipitation of moisture from the atmosphere, may be regarded as in an inverse proportion to the difference between the two thermometers: but in making this estimate, regard must be had to the time of day at which the observation is made. In settled weather, the dryness of the air increases with the diurnal heat, and diminishes with its decline: for the constituent temperature of the vapour remains nearly stationary. Consequently, a less difference at morning or evening, is equivalent to a greater in the middle of the day.

But, to render the observation most completely prospective, regard must be bad at the same time to the movement of the dew point. As the elasticity of the vapour increases or declines, so does the probability of the formation and continuation of rain. An increasing difference, therefore, between the temperature of the air, and the temperature of the point of condensation, accompanied by a fall of the latter, is a sure prognostication of fine weather; while diminished heat, and a rising dew point, infallibly portend a rainy season. When observations shall have been made and registered for a sufficient length of time, the mean results for the different periods of the year will afford accurate standards of comparison, whereby to judge of the state of the vapour: and the three years' journal appended to this Essay, will not be without its use in this respect. In winter, when the range of the thermometer, during

the day, is small, the indication of the weather must be taken more from the actual rise and fall of the point of condensation, than from the difference between it and the temperature of the air. It must be remembered, that a state of saturation may exist, and precipitation even take place in the finest weather, and under a cloudless sky; but this is when the diurnal decline of the temperature of the air, near the surface of the earth, falls below an unfluctuating term of precipitation; and it is probable, that at some period or other of the twenty-four hours, this term is always passed. The radiation of the earth, in the absence of the sun, cools the stratum of air in contact with it; and a light precipitation takes place, of so little density as totally to escape the observation of the eye. At other times it becomes visible, and assumes the appearance of mist or fog. Under such circumstances, the bygrometer will sometimes exhibit a different kind of action. If it be brought from an atmosphere of a higher temperature into one of a lower degree, in which condensed aqueous particles are floating, the mist will begin to form at a temperature several degrees higher than that of the air. The heat emanating from the ball of the instrument, dissolves the particles of water, and forms an atmosphere around it of greater elasticity than the surrounding medium; so that when it is put in action, the point of deposition is proportionably raised. This action does not at all interfere with the determination of the real force and quantity of vapour; for, in all such cases, the full saturation of the atmospheric temperature must have place, and, consequently, the temperature of the vapour must be coincident with that of the air.

This kind of precipitation, which may often be detected by the hy grometer, when it would otherwise escape notice, far from being ins dicative of rain, generally occurs in the most settled weather. It is analogous to the formation of dew, and is dependent upon the same cause, the radiation of the earth, which can only take place under anə. unclouded sky. A sudden change in the dew point, is generally ac companied by a change of wind: but the former sometimes precedes the latter by a short interval: and the course of the aerial currents may be anticipated, before it effects the direction of the weather-cock, or even the passage of smoke. My own experience, and the testimony of others, assure me, that the hygrometer, thus applied, is more to be depended upon than any instrument that has yet been proposed. Even when its indications are contrary to those of the barometer, reliance may be placed upon them; but simultaneous observations of the two most usefully correct each other. The rise and fall of the mercurial column is, most probably, primarily dependent upon the state of the upper regions of the atmosphere with regard to heat and moisture. Local chemical alterations of its density, thus partially brought about, are mechanically adjusted, and the barometer gives us notice of what is going on in inaccessible regions. A rise in the dew point, accompanied i by a fall of the barometer, is an infallible indication that the whole mass of the atmosphere is becoming imbued with moisture, and a copious precipitation may be looked for. If the fall of the barometer take place at the same time that the point of precipitation is depressed, we

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may conclude that the expansion which occasions the former, has arisen at some distant point, and wind, not rain will be the result. But when the air attains the point of precipitation, with a high barometer, we may infer that it is a transitory and superficial effect, produced by local depression of temperature. pp. 149-52.

Into the more deeply scientific application of the Hygrometer as detailed in the essay before us, our limits prevent us from entering. The lover of Meteorological science, and the investigator of natural phenomena in general, will, however, find much valuable and original matter contained in it.

The only other topic to which we shall at present advert, is the instructions laid down by Mr. Daniell for the Meteorologist in his diurnal observations. The proper hours of the day for making these, are indicated by the barometer: the maximum height of the mercurial column is about 9 A. M.;, the mean at 12, and the minimum at 3 P. M. These are the hours which should be selected, if the Meteorologist have opportunities for registering the result three times during the day should circumstances only allow of his making two observations, 9 A. M. and 3 P. M. are the proper periods: if only once, noon should be chosen. Even those who merely consult the barometer as a weather-glass, would, Mr. Daniell thinks, find it advantageous to attend to these hours; for he has remarked, that by much the safest prognostications from that instrument may be derived from observing when the mercury is inclined to move contrary to its periodical course. Should the column rise between 9 A. M. and 3 P. M. it indicates fine weather: should it fall from 3 to 9, rain may be expected. The periods of the barometric observations are. likewise well adapted for those of the thermometer and the hygrometer.

Independently of the Essays which we have noticed, important matter will be found in those which we are compelled to pass over, and which consist of, 1. An Essay on the Constitution of the Atmosphere. 2. On the Radiation of Heat in the Atmosphere. 3. On the Horary Oscillations of the Ba rometer. 4. On the Climate of London. 5. Meteorological Observations at Madeira, Sierra Leone, Jamaica, and other stations between the Tropics, by Captain Sabine, R.A. F.R.S. 5. Meteorological Observations in Brazil, and on the Equator, by Alexander Caldcleugh, Esq. 6. Remarks upon the Construction of the Barometer and Thermometer, &c. 7. Meteorological Observations upon Heights. The volume concludes with a Meteorological Table for three years, kept according to the principles inculcated by the Author.