A notice of all what can increase the progress of human knowledge?
Climate Under the latitude of this query, I will presume it not improper nor unacceptable to furnish some data for estimating the climate of Virginia. Journals of observations on the quantity of rain, and degree of heat, being lengthy, confused, and too minute to produce general and distinct ideas, I have taken five years observations, to wit, from 1772 to 1777, made in Williamsburgh and its neighbourhood, have reduced them to an average for every month in the year, and stated those averages in the following table, adding an analytical view of the winds during the same period.
The rains of every month, (as of January for instance) through the whole period of years, were added separately, and an average drawn from them. The coolest and warmest point of the same day in each year of the period were added separately, and an average of the greatest cold and greatest heat of that day, was formed. From the averages of every day in the month, a general average for the whole month was formed. The point from which the wind blew was observed two or three times in every day. These observations, in the month of January for instance, through the whole period amounted to 337. At 73 of these, the wind was from the North; at 47, from the North-east, &c. So that it will be easy to see in what proportion each wind usually prevails in each month: or, taking the whole year, the total of observations through the whole period having been 3698, it will be observed that 611 of them were from the North, 558 from the North-east, &c.
Though by this table it appears we have on an average 47 inches of rain annually, which is considerably more than usually falls in Europe, yet from the information I have collected, I suppose we have a much greater proportion of sunshine here than there. Perhaps it will be found there are twice as many cloudy days in the middle parts of Europe, as in the United States of America. I mention the middle parts of Europe, because my information does not extend to its northern or southern parts.
By this it may be seen that the South-west wind prevails equally at both places; that the North-east is, next to this, the principal wind towards the sea coast, and the North-west is the predominant wind at the mountains. The difference between these two winds to sensation, and in fact, is very great. The North-east is loaded with vapour, insomuch, that the salt makers have found that their crystals would not shoot while that blows; it brings a distressing chill, is heavy and oppressive to the spirits: the North-west is dry, cooling, elastic and animating. The Eastern and South-eastern breezes come on generally in the afternoon. They have advanced into the country very sensibly within the memory of people now living. They formerly did not penetrate far above Williamsburgh. They are now frequent at Richmond, and every now and then reach the mountains. They deposit most of their moisture however before they get that far. As the lands become more cleared, it is probable they will extend still further westward.
Going out into the open air, in the temperate, and in the warm months of the year, we often meet with bodies of warm air, which, passing by us in two or three seconds, do not afford time to the most sensible thermometer to seize their temperature. Judging from my feelings only, I think they approach the ordinary heat of the human body. Some of them perhaps go a little beyond it. They are of about 20 or 30 feet diameter horizontally. Of their height we have no experience; but probably they are globular volumes wafted or rolled along with the wind. But whence taken, where found, or how generated? They are not to be ascribed to Volcanos, because we have none. They do not happen in the winter when the farmers kindle large fires in clearing up their grounds. They are not confined to the spring season, when we have fires which traverse whole counties, consuming the leaves which have fallen from the trees. And they are too frequent and general to be ascribed to accidental fires. I am persuaded their cause must be sought for in the atmosphere itself, to aid us in which I know but of these constant circumstances; a dry air; a temperature as warm at least as that of the spring or autumn; and a moderate current of wind. They are most frequent about sun-set; rare in the middle parts of the day; and I do not recollect having ever met with them in the morning.
The variation in the weight of our atmosphere, as indicated by the barometer, is not equal to two inches of mercury. During twelve months observation at Williamsburgh, the extremes were 29, and 30.86 inches, the difference being 1.86 of an inch: and in nine months, during which the height of the mercury was noted at Monticello, the extremes were 28.48 and 29.69 inches, the variation being 1.21 of an inch. A gentleman, who has observed his barometer many years, assures me it has never varied two inches. Cotemporary observations, made at Monticello and Williamsburgh, proved the variations in the weight of air to be simultaneous and corresponding in these two places.
Our changes from heat to cold, and cold to heat, are very sudden and great. The mercury in Farenheit's thermometer has been known to descend from 92 degrees. to 47degrees. in thirteen hours.
It is taken for granted, that the preceding table of averaged heat will not give a false idea on this subject, as it proposes to state only the ordinary heat and cold of each month, and not those which are extraordinary. At Williamsburgh in August 1766, the mercury in Farenheit's thermometer was at 98degrees. corresponding with 29 1/3 of Reaumur. At the same place in January 1780, it was at 6degrees. corresponding with 11 1/2 below 0. of Reaumur. I believe (*) these may be considered to be nearly the extremes of heat and cold in that part of the country. The latter may most certainly, as, at that time, York river, at York town, was frozen over, so that people walked across it; a circumstance which proves it to have been colder than the winter of 1740, 1741, usually called the cold winter, when York river did not freeze over at that place. In the same season of 1780, Chesapeak bay was solid, from its head to the mouth of Patowmac. At Annapolis, where it is 5 1/4 miles over between the nearest points of land, the ice was from 5 to 7 inches thick quite across, so that loaded carriages went over on it. Those, our extremes of heat and cold, of 6degrees. and 98degrees. were indeed very distressing to us, and were thought to put the extent of the human constitution to considerable trial. Yet a Siberian would have considered them as scarcely a sensible variation. At Jenniseitz in that country, in latitude 58degrees. we are told, that the cold in 1735 sunk the mercury by Farenheit's scale to 126 degrees. below nothing; and the inhabitants of the same country use stove rooms two or three times a week, in which they stay two hours at a time, the atmosphere of which raises the mercury to 135 degrees. above nothing. Late experiments shew that the human body will exist in rooms heated to 140 degrees. of Reaumur, equal to 347 degrees. of Farenheit, and 135 degrees. above boiling water. The hottest point of the 24 hours is about four o'clock, P. M. and the dawn of day the coldest.
(*) At Paris, in 1753, the mercury in Reaumur's thermometer was at 30 1/2 above 0, and in 1776, it was at 16 below 0. The extremities of heat and cold therefore at Paris, are greater than at Williamsburgh, which is in the hottest part of Virginia.
The access of frost in autumn, and its recess in the spring, do not seem to depend merely on the degree of cold; much less on the air's being at the freezing point. White frosts are frequent when the thermometer is at 47 degrees. have killed young plants of Indian corn at 48 degrees. and have been known at 54 degrees. Black frost, and even ice, have been produced at 38 1/2 degrees. which is 6 1/2 degrees above the freezing point. That other circumstances must be combined with the cold to produce frost, is evident from this also, that on the higher parts of mountains, where it is absolutely colder than in the plains on which they stand, frosts do not appear so early by a considerable space of time in autumn, and go off sooner in the spring, than in the plains. I have known frosts so severe as to kill the hiccory trees round about Monticello, and yet not injure the tender fruit blossoms then in bloom on the top and higher parts of the mountain; and in the course of 40 years, during which it has been settled, there have been but two instances of a general loss of fruit on it: while, in the circumjacent country, the fruit has escaped but twice in the last seven years. The plants of tobacco, which grow from the roots of those which have been cut off in the summer, are frequently green here at Christmas. This privilege against the frost is undoubtedly combined with the want of dew on the mountains. That the dew is very rare on their higher parts, I may say with certainty, from 12 years observations, having scarcely ever, during that time, seen an unequivocal proof of its existence on them at all during summer. Severe frosts in the depth of winter prove that the region of dews extends higher in that season than the tops of the mountains: but certainly, in the summer season, the vapours, by the time they attain that height, are become so attenuated as not to subside and form a dew when the sun retires.
The weavil has not yet ascended the high mountains.
A more satisfactory estimate of our climate to some, may perhaps be formed, by noting the plants which grow here, subject however to be killed by our severest colds. These are the fig, pomegranate, artichoke, and European walnut. In mild winters, lettuce and endive require no shelter; but generally they need a slight covering. I do not know that the want of long moss, reed, myrtle, swamp laurel, holly and cypress, in the upper country, proceeds from a greater degree of cold, nor that they were ever killed with any degree of cold in the lower country. The aloe lived in Williamsburgh in the open air through the severe winter of 1779, 1780.
A change in our climate however is taking place very sensibly. Both heats and colds are become much more moderate within the memory even of the middle-aged. Snows are less frequent and less deep. They do not often lie, below the mountains, more than one, two, or three days, and very rarely a week. They are remembered to have been formerly frequent, deep, and of long continuance. The elderly inform me the earth used to be covered with snow about three months in every year. The rivers, which then seldom failed to freeze over in the course of the winter, scarcely ever do so now. This change has produced an unfortunate fluctuation between heat and cold, in the spring of the year, which is very fatal to fruits. From the year 1741 to 1769, an interval of twenty-eight years, there was no instance of fruit killed by the frost in the neighbourhood of Monticello. An intense cold, produced by constant snows, kept the buds locked up till the sun could obtain, in the spring of the year, so fixed an ascendency as to dissolve those snows, and protect the buds, during their developement, from every danger of returning cold. The accumulated snows of the winter remaining to be dissolved all together in the spring, produced those overflowings of our rivers, so frequent then, and so rare now.
Having had occasion to mention the particular situation of Monticello for other purposes, I will just take notice that its elevation affords an opportunity of seeing a phaenomenon which is rare at land, though frequent at sea. The seamen call it _looming_. Philosophy is as yet in the rear of the seamen, for so far from having accounted for it, she has not given it a name. Its principal effect is to make distant objects appear larger, in opposition to the general law of vision, by which they are diminished. I knew an instance, at York town, from whence the water prospect eastwardly is without termination, wherein a canoe with three men, at a great distance, was taken for a ship with its three masts. I am little acquainted with the phaenomenon as it shews itself at sea; but at Monticello it is familiar. There is a solitary mountain about 40 miles off, in the South, whose natural shape, as presented to view there, is a regular cone; but, by the effect of looming, it sometimes subsides almost totally into the horizon; sometimes it rises more acute and more elevated; sometimes it is hemispherical; and sometimes its sides are perpendicular, its top flat, and as broad as its base. In short it assumes at times the most whimsical shapes, and all these perhaps successively in the same morning. The Blue ridge of mountains comes into view, in the North East, at about 100 miles distance, and, approaching in a direct line, passes by within 20 miles, and goes off to the South-west. This phaenomenon begins to shew itself on these mountains, at about 50 miles distance, and continues beyond that as far as they are seen. I remark no particular state, either in the weight, moisture, or heat of the atmosphere, necessary to produce this. The only constant circumstances are, its appearance in the morning only, and on objects at least 40 or 50 miles distant. In this latter circumstance, if not in both, it differs from the looming on the water. Refraction will not account for this metamorphosis. That only changes the proportions of length and breadth, base and altitude, preserving the general outlines. Thus it may make a circle appear elliptical, raise or depress a cone, but by none of its laws, as yet developed, will it make a circle appear a square, or a cone a sphere.