The Philosophy of the Weather Part 10

Thus, alluding to the winds west of the Mississippi, and between the parallels of 36 and 60, he says:

"On the American continent, west of the Mississippi, there appears to be more diversity in the mean direction of the wind, yet here it is westerly at sixteen stations out of twenty, from which observations have been obtained. The most peculiar feature in this region, is the _line_ of southerly winds on the western borders of Arkansas and Missouri. It seems to form a connecting link between the winds of this zone and the south-easterly ones that we find south of it; and, in some degree, to favor an idea that has been advanced, that there is a vast eddy, extending from the western sh.o.r.e of the Gulf of Mexico, to the eastern sh.o.r.e of the Atlantic; that the easterly trade-winds of the Atlantic Ocean, when they strike the American continent, veer northwardly, and then N. E., and thus recross the Atlantic, and follow down the coast of Portugal and Africa, till they complete the circuit."

This mean prevalence of the curving winds indicates the course of the western portion of the concentrated counter-trade, of which we have so fully spoken, and to which that portion owes its rains and fertility.

Doubtless the curve would have been traced somewhat further west, if observations had been obtained from more westerly stations.

The idea of an eddy, to which Professor Coffin alludes, is of course unsound; that of a counter-trade, most fully confirmed; the curve corresponding with that of the regular rains and fertility as they are known to exist.

Professor Coffin is a believer in the generally-received theory of rarefaction, as the cause of all winds. His work is published by the Smithsonian Inst.i.tution, and the theory is, so far forth, nationalized.

But he found it very difficult to reconcile all the facts he obtained, with the theory, and, possessing a truth-loving mind, he frankly admits it. Alluding to the prevalence of N. E. winds off the coast of Africa in the summer months, as shown by certain numbered wind-roses, he says:

"Nos. 81, 83, 86, and 91, have caused me much perplexity. The arrows for the warmer months evidently indicate a point of rarefaction situated to the _south_ or _south-west_, and yet all the observations from which they were computed were taken within a few hundred miles of the African coast and desert of Sahara; a region, the annual range of whose temperature must be exceedingly great. The only way in which I can account for a fact so astonis.h.i.+ng, is, by supposing the deflecting forces at these numbers to be secondary to the influence which we see so strongly marked in Nos. 88, 89, and 90. Let us, then, first devote our attention to these."

(We have not s.p.a.ce for the map of Professor Coffin, nor is it necessary to insert it. The numbers 81, 83, 86, and 91, refer to respective portions of the Atlantic, west of Africa, North of the Cape de Verdes, of 5 of lat.i.tude each, where the N. E. trades are drawing off from the coast. The Nos. 88, 89, and 90 refer to like portions _below_ the Cape de Verde, where the S. W. monsoons are found under the rainy belt; and the explanation of the distinguished author is an attempt to account for the blowing of the trades _from_ Sahara, by supposing them connected with the monsoons further south, which seem to blow toward it.)

"The intense heat of the Great Desert rarefies the air exceedingly from June to October, inclusive, and hence the arrows of unparalleled length (Plate XII.)," (showing the monsoon winds below the Cape de Verdes,) "pointing toward it during those months, the longest being longer than that which represents the most uniform of the trade-winds, in the ratio of 104 to 89. The influence of this rarefaction is sufficient to curve the powerful current of the trade-winds in the manner exhibited on Plate VII. Nos. 89 and 90, and to produce the not less remarkable change in No. 88, holding the current back and r.e.t.a.r.ding it, so that its progressive motion in the _three_ months of July, August, and September united, hardly exceeds that during any _one_ of the colder months of the year. But while this is so, the trades on the western side of the Atlantic are pursuing nearly their regular track, being but slightly affected by these influences. As a consequence, the latter must leave, as it were, a partial vacuum behind them, which is filled by air flowing in from the north-east and south-east. This will account for the seeming anomaly of having a somewhat strong deflecting force directed toward mid-ocean, in the hottest part of the year, as in the numbers above referred to. _And yet it may be very naturally asked, Why does not the air from these parts supply the Great Desert directly, instead of taking a circuitous route to supply the region that supplies it? A question which, I confess, it seems difficult to answer._"

(The italicization in the foregoing extract is mine).

Here the worthy professor finds a fact inconsistent with the theory of rarefaction--viz.: that the winds blow off sh.o.r.e, and toward mid-ocean, opposite Sahara, and he is "perplexed and astonished." The theory, however, must be maintained, and one of those modifying hypotheses which have made meteorology such a complicated piece of patch-work, must be invented; some "deflecting forces" found. There is the Great Desert, bordering upon the ocean, north of the Cape de Verde Islands, for a distance of six hundred miles, widening as it extends inland, whose temperature, as he says, "_must be exceedingly great_;" and doubtless is so, and yet the air, instead of blowing in upon it in a hurricane, is actually drawing off from it, and blowing towards the S. W., where the water and air do not rise above 84. Well may he be "perplexed and astonished."

Turning south, however, to the distance of five hundred miles or more, he finds the S. W. monsoon winds, which in those months blow under the belt of rains, toward the land, in the direction of, but at a great distance from, Sahara. It is an easy matter to suppose that they reach the Great Desert and supply its vortex of rarefaction, inasmuch as they blow in a direction toward it, and distance is no impediment to supposition.

Then it is necessary to _suppose_ that the S. E. and N. E. trades, at the south-west, draw so strongly to the westward as to create a partial vacuum to the S. W. of Sahara, which is filled by the winds which draw off sh.o.r.e, and then we have the supply brought from the distance of five hundred miles or more, by an ascending vortex, which creates a vacuum, and the air near the vortex taken away in _another_ direction by a _partial_ vacuum; and so an ascending _vortex_, which creates a vacuum is supplied from a distance, and a _partial vacuum_ at a distance is supplied by the air near the perfect vacuum. Such an idea of a supply by a circuitous route, and secondary influence, is not very philosophical, to say the least, and Professor Coffin feels it; and to the question, Why is it so? which, he says, may very naturally be asked, he confesses there is no answer. And there would be none, even if his suppositions were based upon facts. But other questions might be asked equally difficult to be answered, viz.:

1st. Is there any rarefaction which can draw the trades to the west, and in that particular locality, in opposition to the supposed vortex of Sahara, by creating a _partial vacuum_?

2d. Are they in fact so drawn?

3d. Do the S. W. winds, south of the Cape de Verdes, and _under the rainy belt_, which in the summer months extend up to these islands, _reach the desert at all_?

These are pertinent questions, _and every one of them must be answered in the negative_. The hypothesis is without foundation, and Professor's Coffin's perplexity and astonishment must remain, until he abandons the theory of rarefaction entirely. The winds which so perplex him are nothing but the regular N. E. trades, made to originate on the coast and continent of Africa, in summer, by the northern transit of the whole machinery. They not only draw off from the desert coast, but they _blow over the desert itself_ on to the ocean, and into the rainy belt upon the land, as we have already seen, and the supposed vortex of rarefaction does not exist.

That the monsoons do not reach the desert is demonstrated by the tables of Professor Coffin, and to set it at rest we will make the necessary extracts. Commencing with the region from the equator to 5 N., and from 10 to 55 W. longitude, we have the observed winds in proportion, as follows, for July and August--the south-east trades prevailing, inasmuch as the belt of rains is at this season situated further north.

LAt.i.tUDE 0 TO 5, LONGITUDE FROM GREENWICH 10 TO 55.

+-------------------------------------------------------+ Course. July. August. Course. July. August. ------------------------------------------------------- North. 0 0 S. S. W. 54 111 N. N. E. 8 2 S. W. 1 29 N. E. 6 2 W. S. W. 6 19 E. N. E. 27 16 West. 2 9 East. 31 20 W. N. W. 1 6 E. S. E. 120 96 N. W. 1 0 S. E. 216 276 N. N. W. 0 2 S. S. E. 218 443 Calm. 8 4 South. 69 279 --------------------------- Total 768 1,314 +-------------------------------------------------------+

Here, it is evident that the S. E. trades are the prevailing winds, but their course is variable.

Ascending to the region between 5 and 10 north lat.i.tude, and 10 to 55 west longitude, the northern part of which at this season is covered by the rainy belt; we find the monsoon, the S., S. S. W., and S. W. winds, the prevailing ones in August, although the winds are variable, as usual under the rainy belt.

+-------------------------------------------------------+ Course. July. August. Course. July. August. ------------------------------------------------------- North. 19 6 S. S. W. 188 368 N. N. E. 26 11 S. W. 63 94 N. E. 104 32 W. S. W. 73 93 E. N. E. 30 16 West. 33 48 East. 45 29 W. N. W. 30 18 E. S. E. 36 40 N. W. 21 9 S. E. 93 53 N. N. W. 17 13 S. S. E. 225 307 Calm. 109 74 South. 239 514 --------- ------- --------- Total 1,351 1,725 +-------------------------------------------------------+

Ascending to the region of 10 to 15 north lat.i.tude, and 15 to 45 west longitude, we find the winds exceedingly variable, and the monsoons diminished remarkably. If Professor Coffin's theory was correct, they should increase as they approach the desert; but they in fact, diminish, and the N. E. trades are found at the north portion.

+-------------------------------------------------------+ Course. July. August. Course. July. August. ------------------------------------------------------- North. 17 55 S. S. W. 30 71 N. N. E. 64 74 S. W. 33 63 N. E. 155 149 W. S. W. 19 43 E. N. E. 91 71 West. 12 25 East. 83 60 W. N. W. 17 21 E. S. E. 25 26 N. W. 13 24 S. E. 17 26 N. N. W. 24 56 S. S. E. 13 33 Calm. 62 78 South. 9 44 --------- ------- --------- Total 684 919 +-------------------------------------------------------+

Ascending to the region between 15 and 20 north lat.i.tude, and 15 to 45 west longitude, we get north of the belt of rains _and lose the monsoons entirely although still below the desert_; and find the regular N. E.

trades, with less variable winds than are found in almost any other part of the ocean.

+-------------------------------------------------------+ Course. July. August. Course. July. August. ------------------------------------------------------- North. 39 20 S. S. W. 0 5 N. N. E. 210 185 S. W. 0 5 N. E. 112 87 W. S. W. 8 3 E. N. E. 114 104 West. 0 1 East. 20 36 W. N. W. 0 4 E. S. E. 21 17 N. W. 3 4 S. E. 0 2 N. N. W. 3 31 S. S. E. 2 11 Calm 20 8 South. 5 1 --------- ------- --------- Total, 557 526 +-------------------------------------------------------+

Ascending still further to the region between 20 and 25 north lat.i.tude, and 15 and 45 west longitude, which borders, in part, on the S. W.

corner of the desert, and we have not, during the month of August, a single wind between S. S. E. and W. N. W., which blows in upon the land; and _only twelve instances out of three hundred and ninety-four in this hottest month in the year, and on the southern portion of the desert, when the wind blows on sh.o.r.e from any quarter_. This is demonstration. The monsoon winds are confined to the rainy belt; they do not reach the desert, nor does the desert attract the winds from the ocean, or reverse, hold back, or disturb the trades.

+-------------------------------------------------------+ Course. July. August. Course. July. August. ------------------------------------------------------- North. 25 20 S. S. W. 3 0 N. N. E. 210 153 S. W. 2 0 N. E. 129 77 W. S. W. 13 0 E. N. E. 110 86 West. 0 0 East. 8 20 W. N. W. 0 3 E. S. E. 4 11 N. W. 2 1 S. E. 0 3 N. N. W. 5 8 S. S. E. 1 7 Calm. 2 5 South. 1 0 --------- ------- --------- Total, 515 394 +-------------------------------------------------------+

Ascending once more, to the region between the degrees of 25 and 30, north lat.i.tude, and 15 and 45, west longitude, we find it bounded east entirely on the center of the desert. Now here, certainly, there must be evidence of the truth of the rarefaction theory, if any where on the face of the earth. Yet here, in July and August, we find the trades as regular as any where, and not more variable winds than are found in the trades toward their northern limits every where, and in August, only forty out of four hundred and twenty-nine winds, blowing directly or indirectly on sh.o.r.e.

+-------------------------------------------------------+ Course. July. August. Course. July. August. ------------------------------------------------------- North. 32 19 S. S. W. 9 6 N. N. E. 155 125 S. W. 3 9 N. E. 144 35 W. S. W. 13 14 E. N. E. 140 89 West. 12 3 East. 48 57 W. N. W. 7 7 E. S. E. 31 23 N. W. 11 1 S. E. 8 7 N. N. W. 36 6 S. S. E. 8 12 Calm. 18 12 South. 5 4 --------- ------- --------- Total, 680 429 +-------------------------------------------------------+

It would seem to be impossible for any man to believe in the theory of rarefaction, after an examination of these tables.

Professor Coffin discovers other anomalies, for which he finds it difficult to account. Among these are the northerly tendency, in the afternoon, of the winds in Ohio, south of Lake Erie; the winds of south-western Asia, which, he says, "Are so irregular as to defy all attempts to reduce them to system;" particularizing the N. W. at Jerusalem, the westerly at Bagdad, the N. E. at Constantinople, the northerly at Trebizond, etc., etc. Jerusalem has the Mediterranean at the N. W., Bagdad has it at the west, Constantinople has the Black Sea at the N. E., Trebizond N. N. W. and N. E., and the counter-trade, as it pa.s.ses over them, draws its storm-surface wind or sea-breeze, from the quarter where evaporation is greatest, and the atmosphere is most susceptible of electrical inductive influence. Precisely as it draws from the ocean and the eastward, east of the Alleghanies, from the lake region, west of the lakes, and from the northward, south of the lakes, and from the westward, east of them.

This law of attraction will explain, too, the mean prevalence of easterly winds north of the parallel of 60, at the stations named in his work.

Great Bear Lake, Great Slave Lake, and Fort Enterprise, lie east of the Rocky Mountain range which interposes between them and the Pacific, and have Hudson's Bay and other large bodies of water on the east and north.

Hence, easterly winds prevail at these places. At Norway House, on Nelson's River, near the north end of Lake Winnipeg, a large body of water, which stretches off to the south, we find the south wind the prevalent one, especially in December, when the northern and north-eastern waters are frozen up, and the N. E. largely present at all seasons of the year.

At New Hernhut, in winter, when Davis' Straits are covered with floes, the prevailing wind is east, drawn from the warm, open sea east of Greenland, where the Gulf Stream is evaporating. But in June and July, when evaporation is going on over Davis' Straits and Baffin's Bay, the prevailing winds are west and south, and the east winds fall off.

Other stations are equally instructive, but I must forbear.

In relation, however, to the easterly zone of wind, of which Professor Coffin speaks, it should be added that the counter-trade, south of the magnetic pole, in high lat.i.tudes, pursues an easterly course, is near the earth, and attracts an opposite wind as it does on the east and north of the pole, in localities where the surface atmosphere is not peculiarly susceptible to its influence, and, therefore, the _winds are mainly opposite to its course_. Thus, at Melville Island, they are almost all westerly and north-westerly, for there the remnant of the counter-trade is pa.s.sing west around the magnetic pole. These westerly and north-westerly winds are very light, and like the gentle easterly breeze which sets toward the c.u.mulus clouds and summer showers.

Since most of this work was written, I have procured, and read with great pleasure, Lieutenant Maury's "Geography of the Sea." It is a work of great interest, and should be in the hands of every one. The extent of ground covered, however, made it necessary for Lieutenant Maury to introduce much matter not derived from his own investigations. In doing this, he has taken received opinions, and has thereby introduced much heresy. The view he adopts in relation to the monsoons, although the popular one with philosophers, is of that character. He says (page 222):

"Monsoons are, for the most part, formed of trade-winds. When a trade-wind is turned back, or diverted, by over-heated districts, from its regular course at stated seasons of the year, it is regarded as a monsoon. Thus, the African monsoons of the Atlantic, the monsoons of the Gulf of Mexico, and the Central American monsoons of the Pacific, are, for the most part, formed of the north-east trade-winds, which are turned back to restore the equilibrium which the over-heated plains of Africa, Utah, Texas, and New Mexico have disturbed. When the monsoons prevail for five months at a time--for it takes about a month for them to change and become settled--then both they and the trade-winds, of which they are formed, are called monsoons."

Again (-- 476-7):

"The agents which produce monsoons reside on the land. These winds are caused by the rarefaction of the air over large districts of country situated on the polar edge, or near the polar edge, of the trade-winds. Thus, the monsoons of the Indian Ocean are caused by the intense heat which the rays of a cloudless sun produce, during the summer time, upon the Desert of Cobi and the burning plains of Central Asia. When the sun is north of the equator, the force of his rays, beating down upon these wide and thirsty plains, is such as to cause the vast superinc.u.mbent body of air to expand and ascend. There is, consequently, a rush of air, especially from toward the equator, to restore the equilibrium; and, in this case, the force which tends to draw the north-east trade-winds back becomes greater than the force which is acting to propel them forward. Consequently, they obey the stronger power, turn back, and become the famous south-west monsoons of the Indian Ocean, which blow from May to September inclusive.

"Of course, the vast plains of Asia are not brought up to monsoon heat _per saltum_, or in a day. They require time both to be heated up to this point and to be cooled down again. Hence, there is a conflict for a few weeks about the change of the monsoon, when neither the trade wind nor the monsoon force has fairly lost or gained the ascendency. This debatable period amounts to about a month at each change. So that the monsoons of the Indian Ocean prevail really for about five months each way, viz.: from May to September, from the south-west, in obedience to the influence of the over-heated plains, and from November to March inclusive from the north-east, in obedience to the trade-wind force."

What the "trade-wind force" is, Lieutenant Maury tells us in another paragraph, viz.: "Calorific action of the sun and diurnal rotation of the earth"--the received calorific theory. I have already shown, I think, conclusively, that there is no expansion and ascent in the supposed region of calms, which induces, or can induce, the trades; and that, in point of fact, the air on the land is cooler under the belt of rains. But as Lieutenant Maury, whose reputation is national, adopts the theory, I shall be pardoned for copying the following table, showing the difference of temperature at two cities of India, before, after, and while the belt of inter-tropical rains is over them. It will be seen that the temperature is actually less when the belt is there, viz., in July and August, than in April and May. _This should be conclusive upon that point._

+----------------------------------------------------+ Anjarakandy. Calcutta. Months. -------------------- ------------------- Rain. Temp. Rain. Temp. ----------- ---------- --------- --------- --------- M. M. M. M. January, 2,26 26,5 0,0 18,4 February, 2,26 27,7 67,68 21,5 March, 6,77 28,4 24,82 25,6 April, 29,33 29,8 130,84 28,5 May, 175,96 28,6 16,24 29,7 June, 794,05 26,6 575,24 29,3 July, 807,59 25,8 338,38 28,1 August, 572,98 26,0 311,31 28,3 September, 311,31 26,4 254,91 28,0 October, 157,91 26,8 42,86 27,2 November, 65,42 26,9 20,30 23,0 December, 29,33 26,5 0,0 19,2 ----------- ---------- --------- --------- --------- Year, 2955,14 27,2 1928,74 26,4 +----------------------------------------------------+

Anjarakandy is on the Malabar coast, between 12 and 13 north lat.i.tude.

Calcutta in an angle of the Bay of Bengal, at 22 30' north lat.i.tude. The former is in and near the focus of the monsoons, and has a temperature in July (when 18 inches of rain fall), about as low as in December.

In the foregoing table from Kaemptz, the rain is in millimetres, about twenty-five of which make an inch, and the temperature is centigrade, which may be raised to Fahrenheit by adding four fifths of the quant.i.ty and also 32--thus, if the height of the centigrade thermometer be 25, add to this four fifths of 25, which is 20, and also 32, the result is 77. Twenty-five centigrade is therefore equal to seventy-seven Fahrenheit.

Lieutenant Maury is not, and should not be a theorist. He occupies the position, in some sort, of a national _investigator_, and, of course, of national _instructor_. Opinions which emanate from him, or which are endorsed by him, should be accurate. Sooner or later that which he has adopted in relation to the monsoons, and some others, must be abandoned.

In addition to what has already been said, I wish to call his, and the reader's attention, to several other facts and considerations in relation to the monsoons, and particularly those of India.