Washington 10th. April 1805
Men of Science have of late years attended to Meteorological occurrences under the expectation, that from a series of observations, may be deduced some theory beneficial to Society: every hint therefore, leading to this object will I am sure be so well received by you, as to supercede the necessity of an apology for its communication.
Supposing the enclosed specimen of a Meteorological Chart exhibits the different phenomena, in a manner more concise and intelligible than the tabular form in which registers are generally kept, I transmit it to you, as a better judge than I can be of its merit; and because, if it afford the means of a more distinct classification and arrangement of the facts necessary for framing a useful system, it will sooner be perfected by the improvements which your information on the subject may suggest, than by what my slight knowledge affords.
So difficult appears the arrangement of the information furnished by Meteorological journals from the variety of causes operating and to be attended to at the same time—the almost impossibility of connecting them all in the same train of reasoning however accustomed to abstruse speculation the mind may have been & however tenacious the memory is of facts,—that little progress seems to have been made in reducing this branch of science to a regular System. It is so important to the Agriculturalist to ascertain the changes about to take place in the weather from observations to be made on the atmosphere, that the subject has always received a considerable portion of their attention; and men of Science have lately brought to their aid all those inventions which can be the means of furnishing correct data.
Many Gentlemen who have kept records of their observations, seem only to have in view the comparison of Climates, and their influence on the human constitution; attending to the prevalence of particular winds, the temperature and gravity of the atmosphere, the quantity of rain, &ca. at different seasons, in order to obtain general results. In doing this however they have collected a mass of information capable I believe of being digested into a system of still greater utility to mankind. It is only by viewing the operation of all the Natural causes in connection that we can acquire such a knowledge of their effect as will be useful; it being from this combination of causes, and not from their individual action that the phenomena we observe are produced. The state of the atmosphere as it is affected by the quantity of the electric fluid and moisture is seldom noted, perhaps from the difficulty in constructing instruments which shall be capable of measuring it, and which will bear an accurate comparison with other instruments of the same nature.
Considering how much more readily the ideas we receive through the medium of sight and touch are recalled by a linear representation of the objects which caused them, than by words; and, that Charts have with advantage been constructed to facilitate the acquisition of Historic and Chronological knowledge, it suggested the peculiar fitness of this mode of deliniation for registering Meteorological facts and observations. Almost all the information we collect on this Subject may be pourtray’d as it is received: much is acquired by measurement, and can therefore again be deliniated by a graduated scale; and, form a Chart or picture capable of transmitting ideas in a manner bearing much similitude to that in which they were received, and without the intervention of words sometimes doubtful in signification. By a Chart therefore containing scales or measures corresponding with the observations, and where the continuation of a line shall represent the progress of time and the measure of the thing under consideration; and where adjunctive circumstances are denoted by colour, or appropriate Characters, we have in one view all the facts in our possession, and which the mind can have to reason upon; and from which its general deductions are to be drawn.
On these principles I have drawn the Chart, from observations made by Mr. Dunbar near the Mississippi, in the year 1799. Philosophers believe in the Lunar theory of the tides; and Husbandmen have in almost all ages believed in the influence of the Moon on the weather, and on vegetation; I have therefore added the phases of the moon during the period embraced by the Chart, that her attraction, if any, may be noted, or how far it is counteracted by other and more powerful causes.
On examining this Chart an exact coincidence will be found by the depression of the Mercury in the Barometer and the approximation of rain. So true an indication of these changes will the barometer be found, as to deserve the attention of all those whose business is affected by them. The Chart suggests another observation of some apparent importance: that variations in temperature as indicated by the thermometer, occur from the same causes which affect the barometer, but in a contrary manner: the elevation of the mercury in the latter being accompanied by a proportionate decrease of temperature and falling of the Mercury in the former instrument,—and this, independent in a great measure of the quarter from which the wind blows during the unsettled weather of Feby. March & April we observe the greatest variation in the gravity and temperature of the atmosphere, its weight decreasing as the warm season approaches. When the Weather becomes settled, in June and July, and the barometer nearly stationary, something like an atmospheric tide is observed: It may not however arise from the same cause with the tides of the Ocean; for, in a fluid so rare and so elastic as the atmosphere, the effects of expansion and condensation by the solar heat, may be more powerful and observeable than what takes place from the attraction of Gravity.
I am Sir, with the greatest Esteem & respect, yours truly,
DLC: Papers of Thomas Jefferson.