METEOROLOGY. 



791 



ground. With regard to the theory of these bodies 

 being concretions formed in the air, there is one 

 principal objection, viz., that the velocity with which 

 they strike the earth, estimated by the depth to which 

 they have been known to penetrate, is so great as to 

 indicate their having fallen from heights far exceed- 

 ing the limits of the terrestrial atmosphere. The 

 remaining theory, especially that modification of it 

 which conceives these meteoric masses to be terres- 

 trial comets, appears encumbered with fewer difficul- 

 ties than either of the others. The solar comets, it 

 is well known, revolve round the sun in very eccentric 

 orbits. In one part of their revolution, they some- 

 times come so near as almost to strike his body. 

 They then move off, far beyond the orbits of all the 

 planets ; and in some instances are gone hundreds of 

 years, before they return. The earth, it is imagined, 

 in like manner, is furnished with its system of comets, 

 whose size and periods of revolution are proportioned 

 to the comparative smallness of the primary body 

 about which they revolve, and which, like the solar 

 comets, fly off in very elliptical orbits ; and during 

 the greatest part of their circuit are too far distant 

 to be visible, fn their approach to the earth, they 

 fall within our atmosphere; by friction of the air 

 they are heated, and highly electrified, and the elec- 

 tricity is discharged with a very violent report, 

 accompanied with the detachment of a portion of the 

 mass, which descends in fragments to the earth. This 

 hypothesis certainly accounts, in a very happy man- 

 ner, for most of the phenomena attending the fall of 

 aerolites. The velocity of the meteor corresponds 

 with the motion of a terrestrial comet, passing 

 through the atmosphere in an elliptical orbit. A 

 body moving near the earth with a velocity less than 

 three hundred miles in a minute, must fall to its sur- 

 face by the power of gravitation. Tf it move in a 

 direction parallel to the horizon, more than four hun- 

 dred and thirty miles in a minute, it will fly off in the 

 curve of a hyperbola; and will never return, unless 

 disturbed in its motion by some other body besides 

 the earth. Within these two limits of three hundred 

 miles on the one hand, and of four hundred and thirty 

 on the other (some allowance being made for the re- 

 sistance of the air and the motion of the earth), the 

 body will revolve in an ellipsis, returning in regular 

 periods. Now, the velocity of the meteors, which 

 have been observed, has generally been estimated 

 at rather more than three hundred miles in a minute. 

 In some instances it is perhaps too great to suffer the 

 body ever to return; but in most cases, it is calcul- 

 ated to be such as would be necessary in describing 

 the lower part of an elliptical orbit. Various lists 

 of the periods, places, and appearances of these 

 showers of stones have been given from time to time 

 in the scientific journals. One of the latest and most 

 complete is that published in the first volume of the 

 Edinburgh Phil. Journ., compiled partly from a 

 printed list by Chladni,and partly from a manuscript 

 one of Mr Allan, read some years ago at the Royal 

 Society of Edinburgh. 



METEOROLOGY (from piru^f, raised in the 

 air, and \oyos , discourse); the science which treats 

 of the phenomena which occur in the atmosphere, of 

 their causes and effects. Men, in all conditions of 

 society, are led by motives of necessity or comfort to 

 study the indications of the weather in the different 

 appearances of the skies. The mariner, the shep- 

 herd, the husbandman, the hunter, have the strongest 

 motives to examine closely every varying appear- 

 ance which may precede more important changes. 

 The result of these observations forms a body of max- 

 ims, in which facts are often stated correctly, but 

 mixed with erroneous deductions and superstitious 

 notions, such as the credulity of ignorant people 



always renders them ready to adopt. Hence the 

 disposition to refer the. ordinary changes of the wea- 

 ther to the influence of the moon, and even the stars, 

 and to look for signs of approaching convulsions, 

 even in the moral world, in horrid comets and strange 

 meteors. The progress of science, which tends to 

 separate the casual precursors from the real causes of 

 phenomena, refutes these false reasonings, dissipates 

 the empty terrors to which they give rise, and aims, 

 by more patient, long continued, and wide extended 

 observations, to deduce the general rules by which 

 the phenomena of the atmosphere appear to be re- 

 gulated. Meteorology borrows from chemistry hei 

 analysis to determine the composition of the air itself, 

 and of the substances which it contains, and by which 

 it is acted upon ; the manner in which the different 

 processes of evaporation, freezing, thawing, &c., go 

 on, and how they affect the state of the atmosphere; 

 the action of these invisible agents, light, heat, elec- 

 tricity, &c., and their tremendous effects. From phy- 

 sics meteorology takes the mechanical action of these 

 and similar powers and substances, the weight and ve- 

 locity of the air, the laws of the reflection, refraction, 

 and motion of light, &c. By these aids this science 

 explains the formation, fall, or deposition of hail, 

 snow, rain, dew, and frost, (see these articles, and 

 those on Clouds, Evaporation, Freezing, and Caloric); 

 the action of thunder and lightning (see Electricity); 

 the prevalence and properties of certain winds (q. v.) ; 

 the effect of the position of a country and the nature 

 of its surface on its climate and productions (see 

 Climate, Temperature, and Mountains); the nature 

 and causes of meteors (see Meteors, and Meteoric 

 Stones), &c. To prepare the way for these and 

 similar inquiries, it is necessary previously to deter- 

 mine the extent and constitution of the medium in 

 which the phenomena take place (see Air, and At- 

 mosphere), and to indicate with precision, and ob- 

 serve with minuteness and accuracy, its precise con- 

 dition at the time of their occurrence, by philosophi- 

 cal instruments. Some of these have long been 

 known, but others are either of recent origin, or have 

 received a more delicate construction from recent 

 observers. The ordinary observations are generally 

 confined to the weight and temperature of the air 

 (see Barometer, and Thermometer) ; but other data are 

 important, and have of late years received more at- 

 tention than formerly. The dryness or humidity of 

 the atmosphere (see Hygrometer) ; its brightness, or 

 degree of illumination (see Photometer); the tint or 

 shade of the cerulean hue of the sky (determined by 

 the cyanometer, invented by Saussure); the variable 

 disposition to chill the surface of the earth by impres- 

 sions of cold transmitted from the higher regions (de- 

 termined by the aethrioscope), are all to be taken 

 into consideration. The daily evaporation from the 

 ground is to be measured by the atmometer; the 

 quantity of rain which falls is to be registered by the 

 ombrometer, or rain-gauge (q. v.) ; the amount of 

 dew deposited should be observed (see Drosometer), 

 and the direction, force and velocity of the wind in- 

 dicated by the anemometer and anemoscope. (See 

 Saussure's Essais sur f Hygrometrie ; De Luc's Idies 

 sur la Meteorologie ; Cotte's Traite de Miteorohgie ; 

 Lampadius's Grundriss der Atmosphiirologie ; arti- 

 cle Meteorology in the Encyclopaedia Metropolitana 

 (1830, second division) ; Daniell's Meteorological Es- 

 says and Observations. 



The value of a meteorological register depends on 

 the accuracy with which it is kept. The observa- 

 tions should be made in a place rather elevated, and 

 exposed freely on all sides to the aspect of the sky, 

 and should be repeated either at equal intervals 

 during the day and -night, or, at least, at those 

 hours which represent most nearly the mean state 



