Abstract
Fourth Report of the Committee, consisting of Prof. Balfour Stewart (secretary), Profs. Stokes, Schuster, G. Johnstone Stoney, Sir H. E. Roscoe, M.P., Captain Abney, and Mr. G. J. Symons, appointed for the purpose of considering the best methods of recording the Direct Intensity of Solar Radiation.—In their last report the Committee gave a description of a copper inclosure which had been constructed by them. This consisted of a copper cube 3½ inches square outside, the faces of which were of an inch thick. The cube was packed round with felt 9/10 of an inch thick, and the whole was faced outside with thin polished brass plates. Thermometers were inserted into that side of the cube intended ultimately to face the sun, and into the opposite side, by means of which the temperature of these sides could be accurately determined. Finally, a thermometer was placed in the vacant space in the very centre of the inclosure. This last thermometer occupies the position that will ultimately be occupied by the internal thermometer, upon which the sun's rays are to fall through a hole; only at this stage the hole had not been constructed. It is obvious that when the instrument is finally in action, with a beam of solar rays (condensed by means of a lens so as to pass through the hole) falling upon the bulb, this thermometer will be subject to a heating effect from two separate causes, (a) It will, first of all, be subject to radiation and convection from the surrounding inclosure, which is gradually (let us suppose) getting hot through exposure to the sun. (b) It will, secondly, have a beam of solar rays of constant size and of constant intensity (except as to variations arising from atmospheric absorption, seasonal change in the sun's apparent diameter, or change in the sun's intrinsic radiation) continuously thrown upon it through the hole. In fine days when there is no abrupt variation of the sun's intensity the temperature of the internal thermometer will remain sensibly constant, or at least will only vary slowly with the sun's altitude; and this temperature will be such that the heat lost by radiation and convection from the internal hot thermometer will be equal to the heat which it gains from the sources (a) and (b), save as to a small correction, calculable from the slow variation of the temperature of the thermometer. Now, our object being to estimate accurately the intensity of source (b), we must be able, notwithstanding the gradual heating of the inclosure, to determine how much heat the internal thermometer gains from source (a). That is to say, we must be able to tell what would be the temperature of the internal thermometer if the instrument were still made to face the sun, but without any aperture. For the solid angle subtended by the hole at any point of the bulb is so small that we may regard it as a matter of indifference whether there be a hole or not, except as to the admission or exclusion of direct solar radiation. It was suggested by Prof. Stokes that a simple practical method of doing this would be to expose the instrument, without a hole, to an artificial source of heat, such as a fire or a stove, the intensity of which might likewise be made to vary. By this means the conditions of the instrument when facing the sun might be fairly represented. Experiments of this nature were made at Manchester by Mr. Shepherd, acting under the superintendence of Prof. Stewart, and these were reduced by Prof. Stokes. It was ascertained from these experiments that the internal thermometer represented with great exactness the temperature of the cube such as it was 3½ minutes before; in other words, there was a lagging time of the internal thermometer equal to 3½ minutes. We may thus find what would be the reading of the internal thermometer if the balance were perfect between the gain of heat by direct solar radiation and the loss of heat by communication to the environment; and as the latter is approximately proportional to the difference of temperature of the envelope and internal thermometer, and the deviation from exact proportionality admits of determination by laboratory experiments, we have the means of measuring the former. We must bear in mind that the lagging time of the final thermometer may be different from that of the thermometer with which the experiments were made. It was likewise ascertained that the difference between the temperature of the internal thermometer and that of the case need not exceed 20° Fahr., and that a comparatively small lens and hole would suffice for obtaining this result. In consequence of this preliminary information, we have made the following additions to the instrument described in our last report:—(1) We have had it swung like the ordinary actinometers with a motion in altitude and azimuth, and with two moderately delicate adjusting-screws, one for azimuth and another for altitude adjustments. (2) We have had a thermometer centrically placed in the interior. The graduation of the stem is very delicate, and extends from 20° to 120° Fahr., the reading being taken from one of the sides. The bulb is of green flint, and the stem of colourless glass. (3) We have also had a small plate of quartz cut and polished and mounted so as to cover the hole, and to be easily removed and replaced. The object of the plate is to prevent irregularities arising from irregular issue of heated air through the hole, entrance of cooler air blown in by wind, &c., and the choice of material was influenced by the wish to permit of frequent cleaning without risk of alteration by scratching. We ought to mention that as it would be difficult to procure the loan of a good heliostat, and expensive to make, we resolved that in the preliminary experiments the adjustments to keep the sun's image on the hole should be made by the observer. Hence the necessity for the adjusting-screws already described. The Committee suggest that they should be reappointed, and that the sum of £10 be placed at their disposal to defray the expenses of further experiments connected with the instrument.
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Reports. Nature 36, 497–499 (1887). https://doi.org/10.1038/036497a0
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DOI: https://doi.org/10.1038/036497a0