TO MR. THOMAS SKIDMAN.
Monticello, August 29, 1822.
You must be so good, Sir, as to excuse me from entering into the optical investigation which your letter of the 18th proposes. The hand of age presses heavily on me. I have long withdrawn my mind from speculations of that kind; my memory is on the wane. I am averse even to close thinking, and writing is become slow, laborious and painful. I will make then but a single suggestion on the subject of your proposition, to show my respect to your request.
To distinct vision it is necessary not only that the visual angle should be sufficient for the powers of the human eye, but that there should be sufficient light also on the object of observation. In microscopic observations, the enlargement of the angle of vision may be more indulged, because auxiliary light may be concentrated on the object by concave mirrors. But in the case of the heavenly bodies, we can have no such aid. The moon, for example, receives from the sun but a fixed quantity of light. In proportion as you magnify her surface, you spread that fixed quantity over a greater space, dilute it more, and render the object more dim. If you increase her magnitude infinitely, you dim her face infinitely also, and she becomes invisible. When under total eclipse, all the direct rays of the sun being intercepted, she is seen but faintly, and would not be seen at all but for the refraction of the solar rays in their passage through our atmosphere. In a night of extreme darkness, a house or a mountain is not seen, as not having light enough to impress the limited sensibility of our eye. I do suppose in fact that Herschel has availed himself of the properties of the parabolic mirror to the point beyond which its effect would be countervailed by the diminution of light on the object. I barely suggest this element, not presented to view in your letter, as one which must enter into the estimate of the improved telescope you propose. You will receive from the professional mathematicians whom you have consulted, remarks more elaborate and profound, and must be so good as to accept mine merely as testimonies of my respect.
TO MR. GEORGE F. HOPKINS.
Monticello, September 5, 1822.
Sir,—Your letter of August —, was received a few days ago. Of all the departments of science no one seems to have been less advanced for the last hundred years than that of meteorology. The new chemistry indeed has given us a new principle of the generation of rain, by proving water to be a composition of different gases, and has aided our theory of meteoric lights. Electricity stands where Dr. Franklin's early discoveries placed it, except with its new modification of galvanism. But the phenomena of snow, hail, halo, aurora borealis, haze, looming, &c., are as yet very imperfectly understood. I am myself an empiric in natural philosophy, suffering my faith to go no further than my facts. I am pleased, however, to see the efforts of hypothetical speculation, because by the collisions of different hypotheses, truth may be elicited and science advanced in the end. This sceptical disposition does not permit me to say whether your hypothesis for looming and the floating volumes of warm air occasionally perceived, may or may not be confirmed by future observations. More facts are yet wanting to furnish a solution on which we may rest with confidence. I even doubt as yet whether the looming at sea and at land are governed by the same laws. In this state of uncertainty, I cannot presume either to advise or discourage the publication of your essay. This must depend on circumstances of which you must be abler to judge yourself, and therefore I return the paper as requested, with assurances of my great respect.
TO MR. CHILES TERRIL.
Monticello, September 25, 1822.
Sir,—I received on the 20th, your letter of the 13th, on the question what is an east and west line? which, you say, has been a subject of discussion in the newspapers. I presume, however, it must have been a mere question of definition, and that the parties have differed only in applying the same appellation to different things. The one defines an east and west line to be on a great circle of the earth, passing through the point of departure, its nadir point, and the centre of the earth, its plane rectangular, to that of the meridian of departure. The other considers an east and west line to be a line on the surface of the earth, bounding a plane at right-angles with its axis, or a circle of latitude passing through the point of departure, or in other words, a line which, from the point of departure, passes every meridian at a right-angle. Each party, therefore, defining the line he means, may be permitted to call it an east and west one, or at least it becomes no longer a mathematical but a philological question of the meaning of the words east and west. The last is what was meant probably by the east and west line in the treaty of Ghent. The same has been the understanding in running the numerous east and west lines which divide our different States. They have been run by observations of latitude at very short intervals, uniting the points of observation by short direct lines, and thus constituting in fact part of a polygon of very short sides.