Book III. OF THE ATMOSPHERE. 355 



particularly in July and August ; the north-east prevails during January, March, April, 

 May, and June, and is most unfrequent in February, July, September, and December ; 

 the north-west occurring more frequently from November to March, and less so in 

 September and October than in any other months. 



2314. Near Glasgow^ the average is stated as follows : 



Winds. Days. \ 



North-east - - 104 

 South-east - . 47 



2315. In Ireland^ the prevailing winds are the west and south-west. 



2316. The (liferent degrees of motion of wind next excite our attention ; and it seems 

 almost superfluous to observe, that it varies in gradations from the gentlest zephyr, which 

 plays upon the leaves of plants, greatly undulating them, to the furious tempest, calcu- 

 lated to inspire horror in the breast of the most callous. It is also a remarkable fact, 

 that violent currents of air pass along, as it were, within a line, without sensibly agitating 

 that beyond them. An instance of this kind occurred at Edinburgh, where the cele- 

 brated aeronaut Lunardi ascended in his balloon, which was conveyed with great velo- 

 city by the wind at the rate of 70 miles an hour, while a perfect calm existed in the city 

 and neighborhood. 



2317. Causes of wind. There are many circumstances attending the ojjerations of the air, which we 

 term wind, that serve for a basis for well-founded conjectures, and those, united to the result of daily ob- 

 servation, render the explanation of its phenomena tolerably satisfactory. It must be clear to the most 

 common capacity, that as the rays of the sun descend perpendicularly on the surface of the earth under 

 the torrid zone, that part of it must receive a greater proportion of heat than those parts where they fall 

 obliquely ; the heat thus acquired communicates to the air, which it rarefies, and causes to ascend, and 

 the vacuum occasioned by this operation is immediately filled by the chill air from the north and south. 

 The diurnal motion of the earth gradually lessens to the poles from the equator : at that point it moves at 

 the rate of fifteen geographical miles in a minute : this motion is communicated to the atmosphere in the 

 same degree ; therefore, if part of it was conveyed instantaneously from latitude 30, it would not directly 

 acquire the velocity of that at the equator ; consequently, the ridges of the earth must meet it, and give 

 it the appearance of an east wind ; the effect is similar upon the cold air proceeding from the north and 

 south, and this similarity must be admitted to extend to each place particularly heated by the beams of 

 the sun. The moon, being a large body situated comparatively near the earth, is known to affect the 

 atmosphere in its revolutions by the pressure of that upon the sea, so as to cause the flux and reflux of it, 

 which we term tides ; it cannot, therefore, be doubted, that some of the winds we experience are caused 

 by her motion. 



2318. The reffular motion of the atmosphere, known by the name of land arid sea breezes, may be ac- 

 counted for upon the above principle : the heated rarefied land air rises, and its place is supplied by the 

 chill damp air from the surface of the sea ; that from the hills in the neighborhood, becoming cold and 

 dense in the course of the night, descends and presses upon the comparatively lighter air over the sea, 

 and hence the land breeze. Granting that the attraction of the moon, and the diurnal movement of the 

 sun affects our atmosphere, there cannot be a doubt but a westward motion of the air must prevail within 

 the boundaries of the trade-winds, the consequence of which is an easterly current on each side : from 

 this, then, it proceeds that south-west winds are so frequent in the western parts of Europe, and over the 

 Atlantic Ocean. Kirwan attributes our constant south-west winds, particularly during winter, to an oppo- 

 site current prevailing between the coast of Malabar and the Moluccas at the same period : this, he adds, 

 must be supplied from regions close to the pole, which must be recruited in its turn from the countries 

 to the south of it, in the western parts of our hemisphere. 



2319. The variable winds cannot be so readily accounted for ; yet it is evident, that though they seem 

 the effect of capricious causes, they depend upon a regular system, arranged by the great Author of na- 

 ture. That accurate and successful observer of part of his works, the celebrated Franklin, discovered in 

 1740, that winds originate at the precise points towards which they blow. This philosopher had hoped to 

 observe an eclipse of the moon at Philadelphia, but was prevented by a north-east storm, that commenced 

 at seven in the evening. This he afterwards found did not occur at Boston till eleven ; and upon enquiry, 

 he had reason to suppose, it passed to the north-east at the rate of about 100 miles an hour. The manner 

 in which he accounts for this retrograde proceeding, is so satisfactory, that we shall give it in his own 

 words, particularly as his assertions are supported by recent observations, both in America and Scotland. 

 He argued thus : " I suppose a long canal of water, stopped at the end by a gate. The water is at rest 

 till the gate is opened ; then it begins to move out through the gate, and the water next the gate is put in 

 motion and moves on towards the gate ; and so on successively, till the water at the head of the canal is in 

 motion, which it is last of all. In this case all the water moves indeed towards the gate ; but the suc- 

 cessive times of beginning the motion are in the contrary way, viz. from the gate back to the head of the 

 canal. Thus to produce a north-east storm, I suppose some great rarefaction of the air in or near the 

 Gulph of Mexico ; the air rising thence has its place supplied by the next more northern, cooler, and 

 therefore denser and heavier air ; a successive current is formed, to which our coast and inland mountains 

 give a north-east direction." According to the observations made by Captain Cook, the north-east winds 

 prevail in the Northern Pacific Ocean during the same spring months they do with us, from which facts 

 it appears the cold air from America and the north of Europe flows at that season into the Pacific and 

 Atlantic Oceans. 



2320. Other descriptions of winds may arise from a variety of causes. As the atmo- 

 sphere has been ascertained to be composed of air, vapor, and carbonic acid and water, it 

 is well known these frequently change their aerial form, and combine with different 

 substances, and the reverse ; consequently partial winds and accumulations must con- 

 tinually occur, which occasion winds of different degrees of violence, continuance, and 

 direction. 



2321. The principal electrical phenomena of the atmosphere are thunder and lightning. 



2322. Thunder is the noise occasioned by the explosion of a flash of lightning passing 

 through the air : or it is that noise which is excited by a sudden explosion of electrical 

 clouds, whieh are therefore called thunder-clouds. 



2323. The rattling, in the noise of thunder, which makes it seem as if it passed througli arches, is pro- 

 bably owing to the sound being excited among clouds hanging over one another, and the agitated air 

 passing irregularly between them. 



A a 2 



