II. The Bakerian Lecture. Observations upon an unusual hori­zontal refraction of the air ; with remarks on the varia­tions to which the lower parts of the atmosphere are sometimes subject

Abstract

The uncertainty of the refraction of the air near the horizon has long been known to astronomers, the mean refraction varying by quantities which cannot be accounted for from the variations of the barometer and thermometer ; on which ac­count, altitudes of the heavenly bodies which are not more than 5° or 6°, ought never to be made use of when any consequences are to be deduced from them. The cause of this uncertainty is probably the great quantities of gross vapours, and exhalations of various kinds, which are sus­pended in the air near to the earth’s surface, and the variations to which they are subject ; causes, of which we have no instru­ments to measure the effects which they produce, in refracting the rays of light. In general, the course of a ray passing through the atmosphere, is that of a curve which is concave towards the earth, the effect of which is to give an apparent elevation to the object ; and thus the heavenly bodies appear above the horizon, when they are actually below it; but it will not alter the position of their parts, in respect to the horizon, that is, the image of the highest part of the object will be up­permost, and the image of the lowest part will be undermost. The figures, however, of the sun and moon, when near the horizon, will suffer a change, in consequence of the refraction of the under limb being greater than that of the upper ; from which they assume an elliptical form, the minor axis of which is perpendicular to the horizon, and the major axis parallel to it. But a perpendicular object, situated upon the surface of the earth, will not have its length altered by refraction, the refrac­tion of the bottom being the same as that of the top.* These are the effects which arc produced upon bodies at or near the horizon, in the common state of the atmosphere, by what I shall call the usual refraction. But, besides the usual refraction which affects the rays of light, the atmosphere over the sea is sometimes found to be in a state which refracts the rays in such a manner as to produce other images of the object, which we will call an effect, from an unusual refraction. In the Phil. Trans, for 1797, Mr. Huddart has described some effects of this kind, which he has ac­counted for by supposing that, from the evaporation of the water, the refractive power of the air is not greatest at the sur­face of the sea, but at some distance above it; and this will solve, in a very satisfactory manner, all the phenomena which he has observed. But effects very different from those which have been described by Mr. Huddart are sometimes found to take place. These I had an opportunity of observing at Rams­ gate, last summer, on August the first, from about half an hour after four o’clock in the afternoon till between seven and eight. The day had been extremely hot, and the evening was very sultry; the sky was clear, with a few flying clouds. I shall describe the phenomena as I observed them with a terrestrial telescope, which magnified between 30 and 40 times ; they were visible, however, to the naked eye. The height of the eye, above the surface of the water, at which most of the obser­vations were made, was about 25 feet; some of them, however, were made at about 80 feet from the surface ; and it did not appear that any of the phenomena were altered from varying the height of the eye, the general effect remaining the same.