I have said that the barometer has no fair weather standard--the mean of 30 inches at the level of the sea being an _average_ of the _fair weather_ elevations and the _foul weather_ depressions. Its fair weather position, it would seem, must be above the mean, therefore, and as much above as its foul weather depressions are below. But this is not precisely true. Its extreme fair weather range is 31 inches, and it rarely reaches that; while its lowest storm range is down to 28, and is the most often reached of the two. My barometer stands about 40 feet above ordinary high-water mark.
It is not a "wheel," but an open, "scale" barometer, and a perfectly good one. Its most reliable fair weather standard is about 30-30/100 inches. It is its _most common summer, set fair position_, but that position is often at other and different elevations, at other periods of the year, during fair weather. The reader must observe for his own locality, and satisfy himself what the most common set fair position for the barometer is, at the different periods of the year, where he resides. When he has ascertained this, he may apply the following principles to ill.u.s.trate its exceptional action, and in judging of the future of the weather:
1st. _As to its rise before storms._--Supposing it to have been stationary, at or about a set fair position, _for the period_, and for one or two or more days, a very _gradual_ and _moderate_ rise is an indication of continued fair weather; and a _sudden_ and _considerable rise_ is indicative of a storm. If the sudden and considerable rise occurs in the latter part of spring, summer, or early autumn, it indicates a storm of the _first_ or _third cla.s.ses_ described in Chapter X., if in winter, a storm of the _first cla.s.s_ only. If the elevation is _very_ sudden and considerable, the storm will probably be _severe_. The philosophy of this, according to my present apprehension of it, is, that these storms present an _extended easterly front_--_settle very near the earth_--and _have a rapid progress_--thus acc.u.mulating the atmosphere somewhat, in advance of them.
2d. _As to its fall before storms without previous rise._--This is always very regular before the second cla.s.s of storms, or polar belts of showers and storms. It is very fairly exemplified in the table from Reid, on page 329. The barometer, so far as I have opportunity to observe, does not rise from a stationary position on the approach of this cla.s.s of storms. At the commencement of heated, summer, dry terms, my barometer has most frequently ranged at about 30.30, and gradually, but slowly, fallen below 30 inches before the belt of showers arrived, and the term closed. The fourth rule of Dalton (Meteorology, page 183) indicates a similar law in England. It is as follows:
"In summer, after a long continuance of fair weather, with the barometer high, it generally falls gradually, and for one, two, or more days, before there is much appearance of rain. If the fall be sudden and great for the season, it will probably be followed by thunder."
3d. _It falls frequently and considerably without rain._--This is owing to the fact that _all_ regular, periodic efforts at condensation do not result in rain. The second, third, and fourth cla.s.ses of storms described, may not (as we have said) _be sufficiently active to precipitate_, although the _series of phenomena_ (including the fall of the barometer) may be, in other respects, perfect. Such an instance may be found in Reid"s table, on page 329, and on the 11th of the month. But the fall in such cases is not as great, unless the wind be violent.
4th. _It rises during considerable gales._--But these are of the kind so often alluded to--viz., the N. W., in the northern hemisphere, and the S.
W., in the southern; and the _philosophy_ of it has been explained, and is observable.
With these explanations, the reader will be able to understand, and practically apply, the barometric changes, in connection with the other phenomena, in forming an opinion of the weather.
_The thermometer_ is also an auxiliary. It _rises_, during the winter half of the year, in the _advance portion of the storm_, and falls when it pa.s.ses off again; and the reverse is true, as we have seen, when its range is very high in summer. It is, therefore, to some extent, a useful auxiliary, although of minor importance.
_The hygrometer_ is of less importance still. It is not in general use as a practical guide to the changes of the weather, and does not deserve to be.
A question, which has been much mooted, deserves a pa.s.sing notice in this connection--viz., whether our climate has gradually become ameliorated and milder on the eastern part of our continent, since its settlement. I have not s.p.a.ce left for its discussion. Humboldt (Aspects of Nature, page 103) is of opinion that there has been no material change. He says:
"The statements so frequently advanced, although unsupported by measurements, that since the first European settlements in New England, Pennsylvania, and Virginia, the destruction of many forests on both sides of the Alleghanys, has rendered the climate more equable--making the winters milder and the summers cooler--are now generally discredited. No series of thermometric observations worthy of confidence extend further back, in the United States, than seventy-eight years. We find, from the Philadelphia observations, that from 1771 to 1824, the mean annual heat has hardly risen 2.7 Fahrenheit--an increase that may fairly be ascribed to the extension of the town, its greater population, and to the numerous steam-engines. This annual increase of temperature may also be owing to accident, for in the same period I find that there was an increase of the mean winter temperature of 2 Fahrenheit; but, with this exception, the seasons had all become somewhat warmer. Thirty-three years" observation, at Salem, in Ma.s.sachusetts, show scarcely any difference, the mean of each one oscillating within 1 of Fahrenheit, about the mean of the whole number; and the winters of Salem, instead of having been rendered more mild, as conjectured, from the eradication of the forests, have become colder, by 4 Fahrenheit, during the last thirty-three years."
The facts hereinbefore stated show that there is nothing like a _regular_ amelioration; that the seasons differ during the same decade, and different decades. The cold decade, from 1811 to 1820, has not been reproduced. But it may be, and we know not how soon. Since that period there has certainly been a change--for even the cold period from 1835 to 1840 did not equal that from 1815 to 1820, nor indeed those of 1775 to 1780 or 1795 to 1800. But as these variations, so far as we are enabled to judge, depend upon the varying influence of the sun"s rays, and of volcanic action, it is impossible to say that equally cold periods will not return, during the latter half of this century.
If the influence of the sun was constant, and volcanic action regular, two causes would tend to modify the seasons:
1st. The exposure of the surface to a more effective action of the solar rays, by a removal of the forests, and by drainage. That such action would be more effective upon a surface thus uncovered and drained, can not be doubted.
2d. _The movement of the area of magnetic intensity, and the magnetic pole, to the west._--There is such a movement, and its progress can be measured by the increase of declination on the east of it, and its decrease on the west. And the effect of it on climate is unquestionable.
In all probability it has had an influence upon ours; and a removal of that area and pole still further west--60 or 80--would change the location of the concentrated trade, and the Gulf Stream, and restore to Greenland the fertility she once had, and which the Faroe Islands now enjoy. And, on the other hand, its removal as far east of its present position would again depopulate Greenland, and render it again inaccessible. But I can not pursue this subject.
Finally, a.s.sistance may be derived from the occasional, although imperfect, accounts of the state of the weather elsewhere, which the newspapers afford. I have been much indebted to the a.s.sociated Press of New York for intelligence contained in their telegraphic reports.
Occasionally they have been very full and instructive.
On this point, however, there is less of reality in the present than of hope in the future. The time must come when the collection and dissemination of meteorological truth, will be deemed an object of national importance, and national duty. Population is increasing, by immigration and propagation, in a rapidly progressive ratio. There has been great danger that it would outrun agricultural production. A short crop this year would have been disastrous to our prosperity--and the danger was imminent. Every description of business, and every financial circle, felt that fever of anxiety it was so well calculated to induce.
The importance of extended agricultural production, and the dependence of all cla.s.ses upon its success, are now in a greater measure appreciated; and none can fail to see the value of a correct understanding of the weather to the agriculturist, how short-sighted soever they may be, in relation to its direct influence upon their own prosperity and happiness.
Our country is, physically, a most favored one. The facts disclosed or alluded to in this volume show that it is without a parallel on the face of the globe; and our facilities for meteorological observation, and the ascertainment and practical application of meteorological truth, are equally pre-eminent. The great extent and unbroken surface of the eastern portion of the continent; its excessive supply of magnetism and atmospheric currents, and the consequent marked character of the phenomena; the existence and prospective increase of telegraph lines over most of its surface; the h.o.m.ogeneous and energetic character of a population united, upon so large a surface, under one government; the freedom of that government from debt, and the excess of its revenue; the possession of a National Observatory, with a competent philosopher at its head; and a national inst.i.tution, liberally endowed, and adapted to the collection and diffusion of practical and scientific intelligence, give us an opportunity and a capacity for connected observation and investigation, and an ability to profit by it, that no other nation can boast.
We have, too, a just national pride. Our exploring ships have penetrated and made discoveries in both hemispheres, and our travelers have visited successfully every clime; and thus our national interests, and obligations, and pride, demand an organization, practical and permanent, in relation to this subject, and the time will come when we shall have it.
When that time comes--when the present _limited horizon_ of each of us is _practically extended over the entire country_--and when the actual state of the weather over every part of it is known, at the same time, to the inhabitants of every other, and every where _read in the light of a correct philosophy_, prognostication will be comparatively simple and certain; and A PROGRESS will have been made, productive of an amount of pecuniary, intellectual, and social benefit to the people, which can not be overestimated. May it come before the shadows of the night of death have gathered around us, that we may have a more perfect view of that atmospheric machinery which distinguishes our planet from others, and is, with such infinite wisdom, adapted to make it a fit habitation for man!
THE END.
APPENDIX.
Since this work was completed I have received a very valuable publication, ent.i.tled, the "Army Meteorological Register." It is a compilation of the observations made by the officers of the medical department of the army, at the military Posts of the United States, from 1843 to 1854 inclusive, prepared under the supervision of the Surgeon-general, and published by direction of the Secretary of War. To this, there is appended a report or general review of the prominent features of American climatology, so far as the basis afforded by the published observation of the army medical Bureau would warrant positive deduction, by Mr. Lorin Blodget, a distinguished meteorologist, accompanied by temperature and rain charts, for each of the four seasons;--exhibiting the various local differences and peculiarities relative to temperature and precipitation in each.
These local differences and peculiarities and contrasts are deduced and delineated by Mr. Blodget with much ability. He was fettered, however, by the prevailing calorific theories, and the unfortunate practice of grouping the phenomena into means for the seasons, Spring, Summer, Autumn, and Winter, which grouping is arbitrary, and comparatively uninstructive.
Hence, he failed to discover what the tables and summaries most clearly disclose--the principles and system unfolded in the foregoing work.
But the summaries of this register contain observations made at posts in Western and Southwestern Texas, in Kansas and Nebraska, and in New Mexico and California, where there has been a dearth of such observations. .h.i.therto, and enable me to demonstrate, more conclusively, and I think so that none can fail to understand it, the truth of the philosophy I have endeavored to exhibit.
To do this, I will take a _year_,--divide it into two seasons, the periods of northern and southern transit, the only natural and correct division--and note the phenomena in each, as each progresses.
And I will take the year 1854, because that is the last year for which the record of observation is complete; because it had marked peculiarities which are remembered; and because I have alluded to those peculiarities, and those allusions should be confirmed or disproved by the record. Unless I mistake exceedingly, the confirmation will be found signal and convincing.
I have a.s.sumed, pp. 187, 351, that the transits were greater in some seasons than others; that the drought of 1854 was owing to an extreme northern transit, or to an extension west of the concentrated counter-trade, or both, leaving us less supplied with moisture than usual.
In point of fact, it appears from these observations that it resulted from _both_ causes, operating _connectedly_; and the annals of Science rarely furnish a more striking instance of a.n.a.logical inference proved true by subsequent investigation.
Commencing then with the commencement of the northern transit about the 1st of February, we are enabled to trace the then location of our concentrated trade, and its subsequent progress to the north till August, and its influence upon temperature and precipitation. And we can also trace the situation during the same period, of the intervening drought, and the inter-tropical belt of rains, and the extension of the latter north over Florida and the cotton-planting States.
On the 1st of February, 1854, our counter-trade was somewhat more concentrated on its extreme winter curve, over the Southern States, than usual. Its line of excess reached up from Fort Brooke, on the peninsula of Florida, to the northwest, a little east of Pensacola on the gulf, cutting Mount Vernon a.r.s.enal north of Pensacola, and extending thence north-westwardly on to Eastern Louisiana, and curving thence and pa.s.sing N. E. or E. N. E., to the Atlantic, about the waters of the Chesapeake Bay. It thinned out to the west over New Orleans and Baton Rouge, supplying them moderately, but did not extend to the forts of Texas on the west, nor the posts in the Indian Territory at the N. W. It was east of Fort Towson, which is the south-eastern one. It did not reach St. Louis on the north, nor extend north of the Ohio River, as will appear from the tables hereinafter given. The following cut shows substantially its situation on the 1st of February.
[Ill.u.s.tration]
Now, during the month of January, we find the following state of things.
_Under_ this concentrated trade, the temperature was above the mean, even if Forts Monroe and McHenry on the Atlantic are included; but Mr. Blodget discredits their returns, and some others which do not conform to general results. On the west and north of its curving line, both precipitation and temperature were below the mean.
Under the counter trade, we have the following stations, with their actual and mean temperature. I have inserted the temperature for several subsequent months, to show a depression in April.
TABLE I.
-------------------------------------------------------------------------LAT.LON.JAN.FEB.MAR.APRIL.MAY.------------------------------------------------------------------------Fort Moultrie32.4579.5150.8353.0962.7262.7673.35Mean of 28 yrs.50.3652.4158.6865.4473.42Fort Pierce27.3080.2067.9167.3373.0171.1078.41Mean of 5 yrs.62.7564.4269.7773.6376.92Fort Meade28.0182.0063.7563.3370.6468.1076.31Mean of 3 yrs.58.4063.2369.0269.8976.69Fort Brooke28.0082.2862.9462.3670.0670.0777.49Mean of 25 yrs.61.5363.5467.7271.8276.64Fort Myers26.3882.0067.5667.3973.7471.0779.13Mean of 4 yrs.63.3967.9872.1973.8680.13Key West24.3281.4871.7571.9576.5673.8980.84Mean of 14 yrs.66.6868.8872.8875.3879.10Fort Barrancas30.1887.2754.7154.5664.9862.9375.40Mean of 17 yrs.53.6155.5861.8068.5175.45Mt. Vernon Ars"l31.1288.0251.5253.1865.2462.3074.64Mean of 14 yrs.50.4453.6960.2666.8773.92Baton Rouge30.2691.1853.4356.4866.2464.6375.10Mean of 24 yrs.53.4755.0261.9369.3075.60-------------------------------------------------------------------------
------------- JUNE.JULY.
------------- 78.5582.06 79.0181.72 82.0984.16 79.0282.50 79.1080.17 78.2479.76 80.5181.08 79.4680.72 82.3581.91 81.2582.87 83.3483.30 81.6383.00 81.0084.55 80.8082.26 79.1778.90 78.0378.62 80.6180.09 80.5681.81 -------------
It will be seen that the temperature was above the mean in January at every post except Baton Rouge, and there it was at the mean. We shall see hereafter that Baton Rouge was near its western line.
Under this trade during this month, and at the same posts, the fall of rain was as follows, compared with the mean:--
TABLE II.
--------------------------------------------------------------JANUARY.FEBR"Y.MARCH.--------------------------------------------------------------1854.Mean.1854.Mean.1854.Mean.-------------------------------------------------------------- Key West.1.772.862.551.380.514.21Fort Myers.1.153.904.702.160.204.60" Brooke.3.882.206.893.012.443.37" Mead.1.301.072.211.011.851.64" Pierce.3.554.453.402.721.053.01" Barrancas.3.453.875.554.957.215.87Mt. Vernon Ars"l11.016.8012.836.046.224.59Baton Rouge.2.855.265.504.916.154.68Fort Moultrie.3.802.392.842.330.254.06--------------------------------------------------------------