By addition we obtain S (w a2) = 178390, S (w a b) The final equations therefore are ⚫0664 ⚫0696 + •1298 + 1936 178390x +38216 y = + 13.3229, from which we obtain, by elimination, x = +00008711, y = - 2.2448; ⚫00005802. 33° 40′, t = '0001047. The lines of total intensity thus deduced are laid down in the annexed chart for differences of '005, these differences corresponding to intervals of 47.6 geographical miles. It will be seen that their direction diverges widely from that of the lines of dip; and although the position of the two classes of lines may need further correction, it does not seem likely that such correction will have the effect of diminishing, at least by any considerable amount, the divergence. On the Phænomena usually referred to the Radiation of Heat. By HENRY HUDSON, M.D., M.R.I.A. Dublin. THE following paper contains the results of a portion of a series of experiments planned several years since, with a view to an experimental analysis of the phænomena ascribed to the radiation of heat. The apparatus generally employed consisted (as in Leslie's experiments) of cubic tin canisters and differential thermometers, together with a parabolic zinc mirror of 17 inches diameter and 44 inches concavity; this was made with a hollow back to it, and a short projecting pipe at the top for the purpose of filling it with any hot or cold liquid at pleasure. I had also an apparatus for heating or cooling the balls of the differential thermometer previously to arranging the instrument in its proper position before the mirror; my object being to examine the different effects produced on the focal ball under all possible combinations of varying the temperatures of the canister, the mirror, and the thermometer. Having found Leslie's differential thermometer (containing sulphuric acid) to be not sufficiently sensitive where the variations of the temperature were small, I made a differential thermometer for these purposes, into which I introduced sulphuric æther coloured with dragon's blood; I shall therefore speak of this instrument as the "ætherial thermometer," to distinguish it from the common differential thermometer. Having observed, in previous experiments, that the radiating power of a surface covered with black japan varnish was about twelve times greater than that of a metallic surface, I have in the experiments to be detailed merely made use of these two kinds of surfaces, my principal object being to ascertain the nature of radiation, a term which I beg leave to use, whether with regard to heat or cold, without thereby intending to imply any reference to the theory by which the phænomena are to be accounted for. Before proceeding to the more immediate objects of my experiments I may state that I have found Professor Leslie's conclusions on the three following points fully confirmed, viz. 1st, If the canister, the mirror, and the thermometer be all of the same temperature with the air, the focal ball is not affected either by the metallic or the varnished side of the canister: 2nd, If the canister alone be heated, the focal ball is more warmed by the varnished than by the metallic side in the proportion of about 12 to 1 and 3rd, If the canister alone be cooled, the focal ball is more chilled by the varnished than by the metallic side in the same proportion of about 12 to 1. I now proceed to give a few examples of such experiments as have been sufficiently frequently repeated to remove all doubt, from my own mind, of their having been produced by occasional or accidental causes. Experiment 1.-In a close room (temperature 62° Fahrenheit) I placed a large tin screen, 4 feet 6 inches by 2 feet 9 inches, in front of the mirror, at a distance of about 6 feet; and having ascertained the position of the focus, I filled the hollow back of the mirror with water at 200° of Fahrenheit, and arranged the ætherial thermometer so that one of the balls being in the focus the other was equally heated by the mirror, the instrument marking zero. I now placed a smaller tin screen, 24 inches by 17, varnished on one side, about a foot in front of the large screen, with its metallic surface facing the mirror on so doing the focal ball was chilled above one fourth of a degree; and on turning the varnished side towards the mirror the focal ball was cooled 3 degrees below zero. On moving the small screen nearer to the mirror, the chilling effects of both sides increased, and more rapidly than they should have done in reference to the mere diminution of the distance; a fact indeed which may be inferred from the effect of the metallic side of the small screen in its primary position. I also remarked that when either side of the screen was left facing the mirror for any considerable time, its effect began to diminish, evidently from that surface becoming warmer. But the other side being then turned produced its peculiar effect; and the former side also being again (after the lapse of a few minutes) put fronting the mirror was found to produce its full effect as at first. Experiment 2.-The large screen and the thermometer being arranged as before, and the mirror heated to 173°, I substituted for the smaller screen a ten-inch canister filled with water at 59°, the temperature of the room being 55° only; and on repeating the trials, as in the previous experiment, I found that this also acted as a cold body, and the varnished side produced the greater effect just in front of the large screen, however, its effects became very small, the black side only producing a chilling effect of about three fourths of a degree. I then filled the canister with water at 67°, and (in this same position) it now acted as a warm body, and the varnished side most efficaciously. On moving it gradually nearer to the mirror, the effects diminished, and at length altogether ceased, so that the thermometer remained at zero, whether the metallic or the varnished side of the mirror |