Redfield does not seem to have formed any just conception of the _immeasurable power_ of a hurricane, _five hundred miles in diameter_; or of the nature of that _rod_ which the _Almighty must insert in it, to whirl it with such violent and long-continued force_; nor any just conception of the tendency of the whirling ma.s.s, in the absence of his "cylindrical vessel," to fly off, tangentially, into the surrounding air; or of the nature or power of the centripetal force necessary to hold the gyratory ma.s.s in its current, and gather it in involute spirals toward a center. Nor has any other man who has witnessed, or read of mountain-tossed waves; of the largest ships blown down and engulfed; of towns submerged, and vessels carried far inland, and left in cultivated fields, by the subsidence of the sea; of st.u.r.dy forests and strongly-built edifices prostrated; or listened to the howling of the tempest, and felt his own house rock beneath him, been able to conceive of any known form of calorific or mechanical, or other power, acting from a comparatively small center, which could hold such an immense irresistable ma.s.s of whirling air in a circle, and _gather it_ in toward the center in gradually contracting spirals. I confess that, to my mind, it seems little less than a mockery of our intelligence for Mr. Redfield, or Professor Dove, or any other man, how distinguished soever he may be, to tell us that all this is the result of a "tendency to left-wise rotation" of ordinary winds, "coming into each other," or "over-sliding," or "meeting," or "encountering," on this "front," or that, down in Central America, or in the West Indies, or the monsoon region; or to talk of "lateral overflows" from mere gravity; of the ascent of warm air, or the descent of cold strata; of the _resistance of adjacent pa.s.sive air_, or other mere _atmospheric resistances_ in connection with such _awful manifestations of power_. Their explanations of these phenomena are not rational, nor can they be believed by any rational man, who will bestow upon them half an hour of _comprehensive, unbiased reflection_.
Waiving many minor points of great force, for this notice of Mr.
Redfield"s theory is already too much extended for my limits, I am constrained to take issue with him on the fact, and to a.s.sert, unhesitatingly, that in a _majority of instances no such barometric curve exists_.
Doubtless the depression beneath the storm is found, and exterior lateral elevations may also be had by _extending the line into the usual fair weather elevation on each side_, as Mr. Redfield is obliged to do, to get his supposed circle of winds at all. Doubtless, too, the seamen sailing out of a storm, on either _side_, and approaching fair weather, will have a rising barometer. But from _front to rear, on the line of progression_, in tropical storms, the curve does not exist on sh.o.r.e, in this lat.i.tude, oftener than in two, or possibly three, cases in ten; and then only upon a single state of facts--that is, when there is an interposition of N. W.
wind; and this, at some seasons, rarely occurs. An elevation usually occurs before the storm, on its front, if it present an extensive easterly front, as one of these cla.s.ses does, and a _depression is left_ after it has pa.s.sed off, unless a considerable body of N. W. wind interposes, as heretofore stated. But when there is not such interposition of N. W. wind (for W., W. N. W., or even N. W. by W. will not suffice), there is not an immediate rise of the barometer corresponding in rapidity and extent with the fall, and frequently none during the first twenty-four hours of bright, fair weather. Let the reader, if he has access to a barometer, note this fact, for it is obvious and conclusive.
Finally, there are other atmospheric conditions to which the barometric changes are obviously due:
1st. The counter-trade is of a different _volume_, at different times, over the same locality, and hence a difference in the normal elevations of the barometer.
2d. It is at a different _elevation_, at different times, over the same locality. It was so found by the investigations of the Kew Observatory Committee referred to; has been so found by other aeronauts, and may readily be seen by a careful, practiced observer.
It is highest, with a high barometer, in serene weather, when a storm is not at hand; and can sometimes be plainly seen to ascend when a considerable volume of N. W. wind is blowing in beneath, and elevating, simultaneously, the trade and the barometer.
Opportunities occur every year, when the northern edge of the dissolving stratus-cloud is attenuated, and the storm is clearing off in the N. W., with wind from that quarter, and a rising barometer, when its gradual elevation may be observed to correspond with the _volume_ of that wind.
3d. During storms, with a low barometer, the _trade_ and the _clouds run low_. This, too, is clearly observable, especially when the stratus-cloud pa.s.ses off abruptly, very soon after the rain ceases. In such cases the barometer will remain depressed for a considerable time, unless another storm supervenes speedily, or the wind sets in from the N. W.
4th. The _trade, in a stormy state, moves faster_ than when in a normal condition. This is observable during the partial breaks which frequently occur in storms, and at other times. It is also inferable from the more rapid progress of the more intense center, and other intense portions of storms, and the consequent greater depression of the barometer, under such centers or intense portions. (See the storm of Professor Loomis.) It is obvious, also, from the greater rapidity of progress attending the more intense and violent storms which all investigations discloses.
These simple facts explain all the phenomena:
1st. The trade stratum is a continuous unbroken sheet, and its descent must displace a portion of the surface atmosphere. A portion of it is impelled forward, aiding in the precedent elevation of the barometer, and a portion is attracted backward, into the s.p.a.ce from which a like portion had been previously attracted by the pa.s.sing storm cloud, forming the easterly wind.
2d. The increased progress of the stormy portion of the counter-trade occasions an acc.u.mulation in front of the storm, and an elevation of the barometer, and tends also to increase the _depression_ under the spot from which it moves. The latter is, to some extent, counteracted by the thin sheets of surface wind which are drawn in under the stratus from the sides. That which is drawn from the front in successive portions, fills the s.p.a.ce from which like portions had been drawn to the westward, and left behind in a pa.s.sive state by the pa.s.sing storm. Thus, the surface atmosphere of New England may pa.s.s under the entire width of a storm, as a gale; moving now in puffs with great violence, as it pa.s.ses beneath irregular and intense portions of the cloud, and now moderately; and be left, in a pa.s.sive state, in Kentucky, occupying the s.p.a.ce from which the atmosphere had been previously drawn by the same storm, _in like manner_, on to northern Texas.
3d. The nearer the stratus-cloud to the earth, the greater the displacement of surface atmosphere, the lower the barometer, and, ordinarily, the more violent the wind. First, because the same intensity, which, by attraction, brings the trade near the earth, acts with greater force upon the surface atmosphere; and, secondly, the storm winds, which are often most rapid beneath the clouds and above the earth, are likely to be felt with more violence at its surface, where the stratus cloud runs low, especially at sea.
I desire to commend all these facts, in relation to the theory of Mr.
Redfield, to the careful attention and observation of those who, although believers in the theory, are not wedded to it; and who have a sincere desire to understand the phenomena which are continually, and thus far, _mysteriously_, occurring within two or three miles of us, while our knowledge of the distant worlds around us--the science of astronomy--seems almost perfect.
I will return to a further and a careful consideration of the nature of the reciprocal action between the earth and the counter-trade, and the facts bearing upon the question, in another chapter. It is obvious that received theories can not aid us materially in the inquiry.
CHAPTER X.
We are yet ignorant of the true nature of magnetism. We trace its lines, as in the diagrams, upon and around the magnet; but we can only do this with soft iron, or other substance, in which magnetic action may be induced. We know that these lines are currents, or lines of force, for that force produces sensible effects, and we measure it by the movements of the needle. We know that these lines may be _deflected_ by other magnetic bodies, and concentrated upon them. We know that the earth, and the smallest magnets, exhibit properties in common. The poles of the magnet are some distance from its extreme ends--so are those of the earth.
The intensity increases, from the center, or near it, to the poles of the magnet, as shown by its attraction; and the same increase of magnetic intensity, from the magnetic equator to the magnetic poles, or near them, is traced upon the earth.
We know that there are two lines, or rather _areas_, of greater intensity upon the globe. One extending from the American magnetic pole, south-eastwardly, to a corresponding pole in the southern hemisphere; and another, the Asiatic, extending from the Siberian pole to a corresponding southern one, in like manner. We know that, from those lines or areas, the intensity, east and west, on the same parallel of lat.i.tude, decreases each way, to about midway between them. Thus, calling the intensity where Humboldt found the magnetic equator over South America, in 7 1" south lat.i.tude, 1, or unity--the least intensity known is, .706, found at the magnetic equator, over the South Atlantic, and at its most southern depression; and it increases to 1.4 in the West Indies, and to 2.0099 upon one or more points of the North American continent, south of the magnetic pole, and about the meridian of 92. That it is 1.805, at Warren, Ohio, in lat.i.tude 41 16", and longitude 72 57", and decreases to 1.774 at New Haven, Connecticut, in lat.i.tude 41 18". That it is but 1.348 at Paris, nearly one third less than on the same lat.i.tude in some portions of this continent. That the line of equal intensity, or "_iso-dynamic_" line, of 1-8/10, is a closed curve of an oval shape, extending somewhat below 40, in the longitude of Cincinnati, and reaches off nearly to Bhering"s Straits, on the west; rising in a similar manner, though not so abruptly, on the east; including the great northern lakes and a considerable part of Hudson"s Bay. While the iso-dynamic lines of 1-85/100, and 1-875/1000, are smaller ovals, included within the former. Such, at least, is the present belief from such investigations as have been made. (See an article by Professor Loomis, American Journal of Science, new series, vol. iv. p.
192.)
Our subject demands a still closer examination of the elements of magnetism and its a.s.sociated electricities, and their influence upon climate and the atmosphere with a view to the solution of the questions in hand, and we will pursue the inquiry in the present chapter.
Waiving, for the present, any further notice of the fact that the counter-trades are concentrated over, and contiguous to, this area of intensity, for the purpose of examining the magnetic phenomena independently, and intending to return to a consideration of their connection with it, we observe:--That it is now well settled that the iso-geothermal lines, or lines of equal terrestrial heat, are coincident, or nearly so, with the lines of equal magnetic intensity. The points where the magnetic intensity is at a minimum, on the magnetic meridian, are the warmest points of that meridian, and those where it is most intense, the coldest.
The magnetic elements of a place may be computed from its thermal ones.
The laws producing or governing the distribution of one, have an intimate physical relation with those producing or governing the other. Professor Norton ably sums up a discussion of the subject (in the American Journal of Science for September, 1847), omitting the theoretic propositions, as follows:
"1. All the magnetic elements of any place on the earth may be deduced from the thermal elements of the same; and all the great features of the distribution of the earth"s magnetism may be theoretically derived from certain prominent features in the distribution of its heat.
"2. Of the magnetic elements, the horizontal intensity is nearly proportional to the mean temperature, as measured by Fahrenheit"s thermometer; the vertical intensity is nearly proportional to the difference between the mean temperatures, at two points situated at equal distances north and south of the place, in a direction perpendicular to the iso-geothermal line; and, in general, the direction of the needle is nearly at right angles to the iso-geothermal line, while the precise course of the inflected line to which it is perpendicular may be deduced from Brewster"s formula for the temperature, by differentiating and putting the differential equal to zero.
"3. As a consequence, the laws of the terrestrial distribution of the physical principles of magnetism and heat must be the same, or nearly the same; and these principles themselves must have, toward one another, the most intimate physical relations."
The magnetic elements, of which Professor Norton speaks, are the declination, dip, and horizontal and vertical forces or intensities.
I have said, that toward the areas of greatest magnetic intensity, the needle every where declines. So as intensity increases, from the magnetic equator toward the poles, the needle, when so suspended as to permit of the motion, _dips_, inclines downward, and the dip is greatest, on the same parallel, where intensity is greatest. To my mind, the magnetic elements are very intelligible. They are all attributable to attraction, and attraction is greatest where intensity is greatest. There is nothing in the earth or atmosphere to make the needle point northerly rather than in any other direction, except magnetic intensity. Thus, the greater intensity of magnetism near the northern and southern points of the globe, attracts the corresponding ends of the needle in those directions. And, as magnetism increases in quant.i.ty or intensity, and the poles are approached, the attraction increases, and the needle dips more and more, till the focus of intensity and attraction is reached, and then it becomes perpendicular. So magnetism is unequally diffused, meridionally, in or over the earth, and there are two equidistant areas where its quant.i.ty or intensity is greatest. These exert a lateral attraction upon the needle; it yields to this attraction, and hence its declination. If it is carried on to one area of intensity, and to the center of it, it will point to the northern focus of intensity or magnetic pole; and, if carried a trifle further west, it will yield to an eastern attraction, and point directly north. If carried still further west, its declination _east_ will increase. Thus its normal direction is to the pole, on the central focus of intensity, and when it points directly north it is west of the central line of intensity. And thus, it seems to me, all the magnetic elements may be resolved into the one element of attraction by excess of intensity or activity.
This impression is strengthened by the fact that the needle moves to the east in the morning, when the solar rays increase magnetic activity in that direction, and west again, as their influence increases there.
Now, these elements--the declination and horizontal and vertical forces--all these periodical, regular, and irregular variations of magnetic activity, are intimately connected with the variations of atmospheric condition:
First, They show an increase of activity during certain hours of the day, corresponding to, and obviously connected with, the diurnal atmospheric changes.
Second, They show an increase of activity during the northern transit of the atmospheric machinery--an _annual_ variation.
Third, They show an increase in that activity during the latter portion of each decennial period, conforming to the occurrence of solar spots.
And, fourth, _Irregular variations_ of activity, corresponding with the _irregular changes_ of atmospheric condition.
We will examine these results, and in doing so, take those of the element of declination--one answering for all.
The magnetic needle moves to the west in summer, from about 8 A.M. till about 2 P.M., and the extent of its progress, during that period, const.i.tutes the magnitude of its daily variation. It is found that this variation differs in different months, and that it is normally greatest in the summer months, and least in the winter, in the ratio of about two to one. It is further found, that in different years the maximum activity occurs in different months, and that the years differ also, and there is a distinctly marked decennial period, corresponding most remarkably with the decennial maxima of recurring solar spots, as observed by Schwabe. Dr.
Lamont, of Munich, gives us the following table of magnitude of declination there, for the ten years preceding 1851, which clearly exhibits this fact, and also the greater intensity during the northern transit of the atmospheric machinery. He says:
"The magnitude of the variations of declination have a period of ten years. For five years there is a uniform increase, and during the following five years a uniform decrease in the variations. With us the magnetic declination is a minimum at about eight o"clock in the morning, and is greatest at two o"clock in the afternoon. Subtracting the declination at eight o"clock from that at two o"clock, we obtain _the magnitude of the diurnal motion_. From the hourly observations, conducted in this observatory since the month of August, 1840, we ascertain the following to be the magnitude of the diurnal motion for each month separately."
+-------------------------------------------------------------------+Jan.Feb.March.April.May.June.July.Aug.+-------------------------------------------------------------------18413.725.138.4311.4911.4711.4910.079.8618423.654.748.3410.339.319.788.389.0318433.824.086.879.719.2410.149.5710.0818442.813.436.959.538.428.888.389.2818452.204.698.2611.9310.8810.739.4410.4218463.306.949.5312.2712.5811.2111.3711.4918473.306.359.8512.4311.8111.7610.9412.8718486.529.0111.9614.5614.2213.8014.6715.4018497.278.4214.0816.8613.6713.8612.5711.5418505.988.8412.1514.3214.0513.3912.5312.68+-------------------------------------------------------------------+ +----------------------------------------------------+Sept.Oct.Nov.Dec.AutmnSpringYear.& Wint.& Sum.----------------------------------------------------8.786.823.712.895.1210.537.827.727.053.862.815.079.097.038.816.823.822.794.709.597.158.236.543.942.984.448.796.618.827.344.498.345.8910.878.1310.397.825.663.226.0811.258.8112.0611.537.064.707.6311.989.5514.0010.305.783.537.8514.4411.0510.799.125.414.098.0613.2110.6412.649.046.203.457.6113.2710.44+----------------------------------------------------+
The Philadelphia and Toronto observations disclose the same state of facts.
Dr. Lamont, also, in his article, gives us the following table of the magnitude of the variations derived from observations at Gottingen:
+--------------------+Year.Mean of Year.--------------------18359.57183612.34183712.27183812.79183911.0318409.9118418.70+--------------------+
A comparison of these tables, and particularly the latter, with Schwabe"s table of spots, is interesting. There is obviously a greater mean variation when the spots are most numerous. Comparing the two with the tables of Hildreth, in relation to the temperature, from 1830 to 1840, there is, to say the least, a most remarkable coincidence. And there are others equally remarkable.
There are also irregularities of action disclosed by all, in different months of the different years, and of the same year, which are obviously connected with the difference of the seasons; and there are constantly occurring irregularities and disturbances which correspond with the, as constantly occurring, irregular atmospheric phenomena. A wide field is here opened for investigation and research. I have not time or opportunity to pursue it. Enough appears, so far as I have examined, to confirm the belief that magnetism is actively concerned in the production of the varied changes, as well as the normal conditions of the weather.
In what manner does it act? An answer to this requires an extension of the inquiry. The lines of magnetic force are every instant pa.s.sing upward from the earth, _around_ and _through_ us. Their connection with heat is unquestionable. They are intimately a.s.sociated, also, with another equally obvious and intensely active agent--electricity. We speak of this as an independent, imponderable, elementary body, but how little we yet know of it. It is every where, in every thing, easily excited into action, and then traceable to a certain, but limited extent. It is set in motion, and becomes obvious to us, by the chemical action of the acids and metals of a galvanic apparatus. We separate it from the atmosphere by friction and excitation, upon non-conductors, as in the electric machine; by the cleavage of crystals and other exciting operations. We obtain it from magnets, by the magneto-electric machine, and from the lines of magnetic force which are ever pa.s.sing into the atmosphere from the earth, by intersecting them with a movable iron wire, properly insulated. _From the current of magnetism which has pa.s.sed through us from the earth, electricity may thus be separated and collected over our heads._ We set it in motion, and obtain it _by heating_ different metals in connection, or the same metal unequally; and from certain animals--like the torpedo and the gymnotus--whose organization is such as to enable them to evolve it.
In all these cases, and they const.i.tute an epitome of the princ.i.p.al methods by which we obtain it in a distinct form, it is made to flow in currents. When thus obtained, and imprisoned in non-conductors, it may be discharged, and with somewhat different effect, as it is discharged in a ma.s.s, disruptively, as it is called, as from the clouds in lightning, or permitted to flow convectively, in currents, along the wires of a galvanic apparatus, or in heated air, as from the earth to a cloud in the tornado.
It is, moreover, capable of division into positive and negative, and when concentrated or disturbed in one body, it tends to create a similar disturbance or division in a contiguous ma.s.s. To this action of electricity, the term static induction is applied. Thus, a positively electrified body _induces_ a division of the electricity in a contiguous body, if both are insulated or surrounded by a non-conducting medium; the negative electricity of the contiguous body being attracted by, and tending to pa.s.s to, the positive of the adjoining body, and the positive being repelled to the opposite side. That, in its turn, if sufficiently powerful, tends to disturb the electricity of its neighbor, and attract away its negative electricity; or, if the body which contains it is free to move, to attract that. Thus, by the conflicting action of a positive atmosphere, and a negative earth, and perhaps counter-trade, influenced by magnetism and the solar rays, the currents and winds of the atmosphere are produced, the atmosphere moving with exceeding ease and rapidity.
Electricity, excited into currents, or obtained and discharged in either of the methods enumerated, is identical in character, and produces certain well-known effects: