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METEOROLOGY.

of the weather. It is obvious that the same mode,' and even thesame scale, may be made to serve for temperature also, by mark-ing degrees upon the horizontal lines, and changing the appear-ance of the line representing temperature, so as to make it readilydistinguishable from the other curve. There is a correspondencein this climate between the two instruments, which will thus oftenbecome conspicuous. It consists in an elevation of temperatureafter a rise of the barometer, and vice versa: the exceptions tothis occur chiefly at the setting in of frost, and when it rains withthe wind from the eastward. But the most remarkable circum-stance which has thus been brought to light is, an influence whichthe sun and moon exercise over the atmosphere in respect to itspressure; and which is detailed in a series of observations, accom-panied with a chart of this kind, for the year 1798, in the Philoso-phical Magazine, vol. vii. p. 355. The effect of this is, a ten-dency in the atmosphere to gain weight while the moon is passingto either quarter, and vice versa to lose it during the approach offull or new moon. The actual change which on a mean of tenyears is found always to take place at London , amounts only' totwo-tenths of an inch in the barometer, which thus occurs twicein each moon. The apparent influence is often much greater fora considerable time together ( There is something in these move-ments of the atmosphere very much resembling the waves produ-cible in dense fluids. Thus a sudden and great depression in thebarometer is followed by an equally sudden rise, which is oftencarried beyond the poinffrom which the original movement com-menced, and vice versa. In Plate CXIX is given the specimenof the register above noticed.

III. Of Evaporation and Rain .

Evaporation is the principal cause of almost all the meteors ofour atmosphere, and mav be reckoned in a more particular man-ner the effect of heat. Evaporation is a natural process which iscontinually going on, and bv means of which the atmosphere sup-plied with aqueous vapours is w hat furnishes mists, dew, rain, hail,snow, &c. See Evaporation, and Rain . We found reason toconclude, says Dr. Thomson, that the water of the atmosphereexists in the state of vapour. We are indebted to the experimentsof Saussure and De Luc, for much of our know ledge, of the qua-lities of vapour. It is an elastic invisible fluid like common air,but lighter; being to common air, according to Saussure, as tento fourteen, or according to Kirwan as ten to twelve: it cannotpass beyond a certain maximum of density, otherwise the particlesof water, w hich compose it, unite together and form small, hollow,visible vesicles, called vesicular vapour; which is of the sameSpecific gravity with atmospherical air. It is of this vapour thatclouds and fogs are composed. This maximum increases with thetemperature, and at the heat of boiling water is so great, that steamcan resist the whole pressure of the air, and exist in the atmospherein any quantity. Evaporation, at least in our climate, is aboutfour times greater during the summer than the winter half-year:other things being equal, it is so much more abundant, the greaterthe difference is between the temperature of the air and of theevaporating surface; so much the less, the nearer they approachto the same temperature, and least of all, when they actually ar-rive at it. Whenever the atmosphere is more than fifteen degreescolder than the evaporating surface, little evaporation takes placeat all. Evaporation is powerfully promoted hv winds, especiallycold winds blowing into warm countries, or warm winds blowinginto cold countries. Tracts of land covered with trees or vegeta-bles emit more vapour, than the same space covered with water.”Dr. Thomson, after quoting the experiments of Dr. Hales, Dr.Watson , and others, upon this subject, for which we refer thereader to the article Evaporation , has this general remark, that,“ the formation of clouds and rain cannot be accounted for by asingle principle with which we are acquainted. It is neither owingto the saturation of the atmosphere, nor the diminution of heat,nor the mixture of airs of dilferent temperatures, as Dr. Iluttonsupposes; for clouds are often formed without any wind at alleither above or below' them ; and even if this mixture constantlytook place, the precipitation, instead of accounting for rain, wouldbe almost imperceptible.” Elem. Client. Vol. III. p. 321. Wemust, therefore, in accounting for the phenomena of evaporationand rain, admit the joint agency of different principles. The wa-ter being raised into the atmosphere, by means of evaporation, is

! there sus|>onded, and by its condensation or precipitation producej the phenomena of clouds. The clouds or forms assumed by t‘ lsuspended water in the interval between (he first precipitation an 1the descent of rain, afford a copious field of observation. I h cseare not, as might be lustily supposed, the sport of winds, changingwith every movement of the containing medium, Indeed tj ieatmosphere, at the height where clouds usually appear, is und* s "tnrbed by the various obstacles which throw it into 'contend* 11 ?streams and eddies near the surface of tiie earth, and flows in 3more direct and even current. Accordingly', the particles of "'a"ter which it contains are allowed to assume a certain arrangement;and constitute a form, which is often equally well defined at a dis-tance with that of solids. From observations relative to this sub'ject, Mr. Howard was led lo conclude thatthp forms or modified'tionsof the clouds are subject to certain fixed laws, in their pjf°‘duction, their action on each other, and their resolution into ra** 1 -And the better to describe these modifications he formed a nom e11 'clature, which w’e shall here quote, and refer the reader for a fuf"ther account of this subject to the Philosophical Magazine, ^

Hi, and 17. There are three simple and distinct modifications, 'J 1any one of which the aggregate of minute drops, called a cmaybe formed, increase to its greatest extent, and finallyde'crease and disappear. By modification is to he understood simp')the structure or manner of aggregation, not the precise fort" ° rmagnitude, which indeed varies every moment in most cloud* 1The principal modifications are commonly as distinguishable fr 01 ”each other as a tree from a hill, or the latter from a lake; althoughclouds in the same modification, considered with respect to eachother, have often only the common resemblances which ex' 1 *among trees, hills, or lakes, taken generally. Thu same agg |-c *gate, which has been formed in one modification, upon a chiU’S®in tlie attendant circumstances may pass into another. Or it i* ia ).continue a considerable time in an intermediate state, partaking fthe characters of two modifications; and it may also disappear nlthis stage, or return to the first modification. Lasllv, aggregate- 1 'separately formed in dilferent modifications, may unite and |* a ”into one, exhibiting dilierent characters in dilierent parts; p r ‘portion of a simple aggregate may pass into another modificafi ol Jjwithout separating from the. remainder of the mass. Hence, tug*'’ther with the simple, it becomes necessary to admit intermedia*,and compound modifications, and to impose names on such 0them as are worthy of notice. The simple modifications arenamed and defined : See Plate CXIX. 1. Cirrus, Def. Nub 1 'cirrata, tenuissima, qua: nndique crescat. Parallel, flexuous 0diverging fibres, extensible in any or in all directions. 2. Cumu' lt 'Def. Nubes cumuluta, densa, sursum orescens. Convex or cnical heaps, increasing upward from a horizontal base. 3.

Def. N uhes strata, aquae mode ex'pansa, deorsum crescens. 'widely extended, continuous, horizontal, sheet, increasing h’°'below. 'Phe intermediate modifications which require to be 3ticed are : 4. Cirro-cumulus. Def. Nubecula densiores, sitb 1 ^tundx, et quasi in agmine apposita:. Small, well defined,ish masses, in close horizontal arrangement. 5. Cirro-str0‘ 1 .^Def. Nubes extenuata , subconcava vel undulata. Nub*’ 1 ' 11 .’,,hujusmodi apposita'. Horizontal or slightly inclined masses, 3nuated towards a part or the whole of thfcir circumference, f ,cave downward ; or undulated, separate, or in groups, consis'L,of small clouds, having these characters. The compound n* otcations are: ti. Cu/niilo-strnh/s. Def. Nubes densa, basim etn" g jcum structure patente exhibens. A dense cloud with the ba* 1 ^the cumulus, hut in its upper part extended into a broad sstructure. 7. Cumulo-cirro-stratus, vel nimbus. Def. N 11 ^vel nubium congeries pluviam eHundens. The rain~cb>H"‘ ^cloud, or system of clouds, from which rain is falling. It is 3 ^rizontal sheet, above which the cirrus spreads, while the cun Jtenters it laterally, and from beneath.

IV. Of the Electricity of the Atmosphere. ^“ The electrical state of the atmosphere, (says Dr. '1“ is a point of considerable importance, and lias, with grea* lp f ,prietv, occupied the attention of philosophers ever sine 1 -Franklin demonstrated that thunder is occasioned by the ag ( ,.j.of electrieity.” “ There are four sources of atmospheric el 1city known: 1. Friction: 9. Evaporation: 3. II J

Cold: 4. Expansion and Contraction: see these art 1 j)()t

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