262 



SCIENCE OF GARDENING. 



Part II. 



most superfluous to obserre^ tliat it varies in gradations from the gentlest zephyr, which 

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

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

 lent currents of air pass along, as it w^ere, within a line, without sensibly agitating that 

 beyond them. An instance of this kind occurred at Edinburgh, where the celebrated 

 aeronaut Lunardi ascended in his balloon, which was conveyed with great velocity by 

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

 neighborhood. 



1260. Causes of wind. There are many circumstances attending the operations of the air, which we term 

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

 ation, 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 de- 

 gree ; 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 ap- 

 pearance of an east wind ; tlie 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. 



1261. The 7-egiilar motion of the attnosphere, known by the name of land and sea breezes, may be accounted 

 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 atmo- 

 sphere, 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 opposite current 

 prevailing between the coast of Malabar and the Moluccas at the same period : this, he adds, must be sup- 

 plied from regions close to the pole, which must be recruited in its turn from the countries to the south of 

 it, in tlie western parts of our hemisi)here. 



1262. 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 nature. 

 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 

 £he 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 retrogade proceeding is so satisfactory, tliat 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 successive times of beginning 

 the motion are in the contrarj- 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." Ac- 

 cording 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 Ame- 

 rica and the north of Europe flows at that season into the Pacific and Atlantic Oceans. 



1263. Other descriptions of winds may arise from a variety of causes. As the atmosphere 

 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 vsrinds and accumulations must continually occur, 

 which occasion winds of different degrees of violence, continuance, and direction. 



1264. T/ie priticipal electrical phenomena of the atmospliere are thunder and lightning. 



1265. Thunder is the noise occasioned by the explosion of a flash of lightning passing 

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

 clouds, which are therefore called thunder-clouds. 



The rattling, in the noise of thunder, which makes it seem as if it passed through arches, is probably 

 owing to the sound being excited among clouds hanging over one anothCT, and the agitated air passing ir- 

 regularly between them. 



The explosion, if high in the air and remote from us, will do no mischief; but when near, it may, and 

 has, in a thousand instances, destroyed trees, animals, &c. This proximity, or small distance, may be esti- 

 mated nearly by the interval of time between seeing the flash of lightning and hearing the report of the 

 thunder, estimating the distance after the rate of 1142 feet for a second of time, or 3| seconds to the mile. 

 Dr. Wallis observes, that commonly the difference between the two is about seven seconds, which at tlie 

 rate above-mentioned, gives the distance almost two miles. But sometimes it comes in a second or two, 

 which argues the explosion very near to us, and even among us. And in such cases, the Doctor assures 

 us, he has sometimes foretold the mischiefs that happened. 



Season of thunder. Although in this country thunder may happen at any time of the year, yet the 

 months of July and August are those in which it may almost certainly be expected. Its devastation is of 

 very uncertain continuance ; sometimes only a few peals will be heard at any particular place during the 

 whole season ; at other times the storm will return at the interval of three or four days, for a month, six 

 weeks, or even longer ; not that we have violent thunder in this country directly vertical in any one place 

 so frequently in any year, but in many seasons it will be percej^tible that thunder-clouds are formed in the 

 neighbourhood, even at these short intervals. Hence it appears, that during this particular period, there 

 must be some natural cause operating for the production of this phenomenon, which does not take place at 

 ether times. This cannot bo the mere heat of the weatiicr, for we have often a long tract erf" hot vreether 



