S62 iSCIEKCE OF AGRICULTURE. Part II. 



In London the mean temperature is 50 ; at Rome and at Montpellier, a little more than 

 60 ; in the island of Madeira, 70 ; and in Jamaica, 80. 



2380. IFind. Were it not for this agitation of the air, putrid effluvia arising from the 

 habitations of man, and from vegetable substances, besides the exhalations from water, 

 would soon render it unfit for respiration, and a general mortality would be the conse- 

 quence. The prevailing winds of our own country, which were ascertained by order of 

 the Royal Society of London, at London, are. 



Days. 



- 18 



- 16 



The westerly winds blow more upon an average in each month of the year than any other, 

 particularly in July and August ; the north-east wind 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. 



2381. ^ear Glasgow, the average is stated as follows : 



2382. In Ireland, the prevailing winds are the west and south-west. 



2383. The different degrees of motion of wind next excite our attention ; and it seems 

 almost superfluous to observe, that it varies in gradation from the mildest zephyr, which 

 plays upon the leaves of plants, gently 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 the fury of the wind being bounded " by a line" occurs 

 in the hurricane of America ; where its devastating course is often accurately marked in 

 the forests for a great extent in one direction. 



2384. Causes of wind. There are many circumstances attending the operations of the 

 air, which we term wind, which serve for a basis for well-founded conjectures, and 

 those, united to the result of daily observation, render the explanation of its phenomena 

 tolerably satisfactory. 



2385. 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 

 chili air from the north and south. The diurnal motion of the earth gradually lessens to the poles from 

 the equator, at which point it moves at the rate of fifteen geographical miles in a minute, and this motion 

 is communicated to the atmosphere in the same degree ; but if part of the atmosphere were conveyed 

 instantaneously to the equator from latitude 3CP, it would not directly acquire the equatorial velocity ; 

 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 ; and this, and the continual shifting of 

 the point of the earth's surface over which the sun is vertical, to the west, are given as the causes of the 

 tides and of the trade winds. The moon's revolutions, by pressing the atmosphere upon the sea, cause 

 the flux and reflux which we call tides ; it cannot, therefore, be doubted, that some of the winds we 

 experience are caused by the moon's motion. 



2386. The regular motion of (he atmosphere, known by the name of land and sea breezes, may be 

 explained by the effects of rarefaction : the air heated over the land rises up, because rarefied, and its place 

 is supplied by the cooler air which flows in from the sea ; this produces the sea breeze ; at sunset, the 

 equilibrium is first restored; but as the earth cools faster by radiation than the water, the air over it 

 becomes cooler than that over the sea, especially if there be mountains in the vicinity ; the air over the 

 land then displaces the light air from the sea, and thus the land breeze is formed. Granting that the 

 attraction of the moon and the diurnal movement of the sun affect our atmosphere, there cannot be a 

 doubt but a westward motion of the air must prevail within the boundaries of the trade-winds, the con- 

 sequence 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 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. 



2387. 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 174(), 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 : " 1 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 

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

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



