THE POPULAR EDUCATOR. 



bined with its motion towards the equator, produces the north- 

 east and south-east trade-winds. These and other regular winds 

 are best observed in the ocean far away from land, as that always 

 interferes, more or less, by creating fresh currents. It is said 

 that a difference is caused in the wind by the proximity even 

 of a coral reef or small island. 



Now it is manifest that there must be some return current 

 for the air, as otherwise it could not continue to flow constantly 

 in the same direction, and it is found that this return current 

 is above the ordinary one, and is in the contrary direction. It is 

 difficult to afford full proof of this, but several facts show that 

 such is the case. Eather more than fifty years ago an eruption 

 took place of the volcano in St. Vincent, and a large quantity 

 of ashes and scoria3 was thrown out to a considerable elevation. 



A portion of this fell and covered the island of Barbadoes. 

 Now the trade-wind blows directly away from Barbadoes ; and 

 it seems certain, therefore, that the ashes were thrown up high 

 enough to reach the upper or return current, and were carried 

 along with that. 



Humboldt states, likewise, that when he ascended the Peak 

 of Teneriffe he found a very strong west wind, while the ordinary 

 north-east trade-wind was blowing on the earth below at the 

 same time. We see, then, that there is a system of convection 

 being carried on in the air on a gigantic scale, the air, heated 

 by the tropical sun, ascending and making room for colder air 

 from temperate regions. As the fact of the sun being vertical 

 over parts of the torrid zone is thus the cause of these winds, 

 we should expect to find that when the sun is north of the 

 equator they extend further north than when the sun is in the 

 southern hemisphere, and such is really the case. The variations 

 in their limits thus caused are usually laid down on maps which 

 show the prevailing winds. 



After the current of heated air that ascends from the equator 

 has travelled some distance, it becomes cooled down by radiation 

 into space, so that its temperature falls below that of the under- 

 current. They therefore change places, and the return current 

 from the equator blows on the surface of the earth, producing 

 south-west winds in the northern hemisphere, and north-west 

 in the southern. The west direction of these arises from the 

 fact that they come from a portion of the earth where the 

 motion is more rapid, and they therefore travel faster than the 

 part they blow over. 



The latitude at which this current descends is about 30. It 

 varies, however, considerably in different parts owing to local 

 causes, and therefore the winds in the temperate zone have not 

 the same degree of regularity as they have nearer the equator. 

 We find, however, by records kept at various places, that if we 

 measure the duration and intensity of the wind in places in the 

 north temperate zone, the prevailing direction is south-west or 

 south-south-west. Still further north, this current, which has 

 become heated by contact with the earth, again ascends and 

 becomes the upper current. 



In the Indian Ocean the effect of the trade- winds is but little 

 felt, but they are replaced by the monsoons. These are 

 periodical winds, and extend from a few degrees south of the 

 equator to the northern shores of the Indian Ocean. Their 

 direction from April to October is south-west, and from October 

 to April north-east. At the periods of change violent storms 

 are experienced, by which great damage is frequently done to 

 shipping and property. This period usually lasts nearly a 

 month. 



The explanation of these phenomena is found in the fact that 

 when the sun comes north of the equator, the plains of India 

 and the surrounding countries become intensely heated by the 

 almost vertical rays of the sun. The air over them, therefore, 

 becomes rarefied and ascends, creating a strong current towards 

 those parts from the southward, for the sea does not become 

 so heated by the sun's rays as the land does, but preserves an 

 almost uniform temperature. When the sun crosses the equator 

 southward, the plains of Australia and the islands near it be- 

 come similarly heated, and thus a reverse current is produced. 

 These winds are, however, somewhat modified by the nature of 

 the country and other causes, which are not yet fully under- 

 stood. For purposes of commerce they are found almost more 

 useful than the trade-winds, as a vessel can proceed to a port 

 during one monsoon, wait there to discharge her cargo, and 

 re-load in the interval of the change, and return with the con- 

 trary wind. 



The land and sea breezes are the next winds which we must 

 notice. These are more felt in lower latitudes than in ours ; but 

 most visitors to the seaside in our own country have doubtless 

 noticed them. A short time after sunset a gentle breeze sets 

 off the coast towards the sea, and continues to blow during the 

 evening and night. After the sun has risen again, this wind 

 ceases, and a contrary one sets in from the sea, mitigating the 

 intense heat which would otherwise be felt. This breeze con- 

 tinues during the day. These phenomena are said to be well 

 seen in the neighbourhood of Vesuvius, where during the day 

 the column of smoke blows inland, but at evening it veers round 

 till it points exactly in the contrary direction. 



These winds are easily accounted for. The sea, as we have 

 already remarked, preserves a nearly uniform temperature, 

 being little affected by the heat of the sun. The land, on the 

 contrary, becomes rapidly heated by day, and at night parts 

 with its heat as rapidly by radiation. The consequence is that, 

 as soon as the sun has been shining a little time, the land attains 

 a higher temperature than the sea ; the air over it therefore 

 rises, and colder air flows in from the sea to fill its place. In 

 the evening this extra heat soon passes off, and the land becomes 

 colder than the sea. The contrary effect then ensues : the air 

 rises over the sea, and a breeze springs up from the land. A 

 simple experiment will serve well to illustrate these phenomena. 

 Fill a tub or other large vessel with cold water, and float on it 

 a saucer filled with warm water. This will represent the island 

 when heated during the day. If we now hold a smoking taper 

 or piece of paper near it, we shall see that a current of air is 

 setting in towards the saucer. Now reverse the experiment, 

 and fill the large vessel with warm water and the small with 

 cold, and we shall find that the current of air flows over the 

 edge of the saucer and drives the smoke away, just as at even- 

 ing the wind sets off the land. 



There are many other periodical winds, but as for the most 

 part they are confined to small areas and only known by local 

 names, we need not further allude to them, except to say that 

 nearly all may be accounted for on the same principles as those 

 we have been considering. 



The winds frequently met with on the borders of desert re- 

 gions, and known in Arabia as the Simoom, in the neighbour- 

 hood of the Mediterranean as the Sirocco, and by other names 

 in other places, owe most of their dangerous character to their 

 dryness, and to the amount of fine dust they carry with them. 

 Sweeping over large tracts of arid sand, they become intensely 

 hot and dry, and often cause the death of those who are unable 

 to obtain shelter from their influence. 



If we thus trace the quarter from which any wind blows, we 

 shall usually be able to understand the reason of its special 

 character. Thus, in England, rain usually comes with a south- 

 west wind, the reason being that this wind, coming from lower 

 latitudes, is warm, and therefore absorbs much water as it passes 

 over the sea. When it reaches our shores it becomes cooled, 

 and a portion of its vapour is at once condensed and falls in 

 the form of rain. 



In mountainous districts a current of air frequently impinges 

 against the cold tops of a mountain, and becoming in this way 

 suddenly cooled, rushes violently down the side, often giving 

 rise to violent and destructive hurricanes. The Pamperos, or 

 winds which rush from the Andes with great violence across 

 the plains of South America, owe their origin to this cause ; 

 and in our own lake districts in Cumberland a somewhat 

 similar effect, though on a much smaller scale, may at times be 

 noticed. 



The whole subject of the varying phenomena of the atmo- 

 sphere is one of great importance and interest ; but we cannot 

 further enlarge upon it here. 



ANSWERS TO EXAMPLES IN LESSON VII. (Page 38.) 



1. The pressure is 5 x 6 '25 x 29'04 x O'ifll pounds, which, is equal 

 to 445-5825 pounds. 



2. Since the volume occupied by the air is only -^A, i.e., - 9 ^, of its 



original volume, the pressure is increased The mercury there! ora 

 stands ? - x 29 inches, or 64f above the level in the other limb, or 



69}| above 0. 



3. 238'21 cubic inches. 



4. Just over 1461 cubic inches. 



5. The temperature to which it is raised is 338. 



6. 128-09 cubic inches. 



