cool enough to handle, and puts the lower edge of each, one by one, into the list-pot, which is the vessel that was before described as containing a very small quantity of melted tin, and the same as that which I have marked No. 5. When the wire of tin is melted by this last immersion, the boy takes out the plate, and gives it a smart blow with a thin stick, which disengages the wire of superfluous metal, and this falling off, leaves only a faint stripe in the place where it was attached. This mark may be discovered on every tin plate which is exposed for sale; the workmen, in the manufactory of them, call it the list. Nothing now remains but to cleanse the plates from the tallow. This is done by means of bran, and as they are cleansed they are put into strong wooden boxes, or boxes of sheetiron, made exactly to fit them; and this completes the whole business. ART. XXVI.-Remarks on Solar Light and Heat. By BADEN POWELL, M. A. [Annals of Phil.] (Continued from page 159.) (27.) In the conclusion of a former paper I alluded to some further experiments which were to follow, relative to the question of the proportion obtaining between the heating and illuminating effects of the solar rays. The method of experi menting alluded to is one which I have not been able to apply to any extensive series of different intensities. It consists in comparing the effect produced on a blackened thermometer by the focal light of different lenses, with the relative calculated intensities of the rays in those foci. Thus we may ascertain whether at these high intensities the same proportion is maintained. Without proof we cannot assume that it is; and a very few comparisons may be sufficient to show, whether the proportion is nearly preserved, or whether there be any considerable deviation from it. (28.) When thermometers are exposed to the action of radiant matter there are several considerations to be attended to in comparing their observed risings: and it will be convenient in the first instance to bring these considerations into one point of view. A thermometer exposed to radiant matter absorbs heat only on one half of its surface, while the other half is radiating again its acquired heat, and the observed effect depends upon the equilibrium which obtains between them. In particular cases only part of one surface may be exposed to heat; the difference between such part and the whole surface, together with the absorbing and radiating powers of the surface, must, therefore, be taken into consideration, as also the rate of communication of heat dependent on the mass. Let the portion of the surface of the bulb exposed to radiant matter, Diameter of the bulb, Its surface, The observed rise in a given time The power of the coating for absorbing heat (of whatever kind) And for radiating it The intensity of heating power The general formula easily deduced on the above considerations will be, h And if the coating be the same, it = and if h―h, it = 1. h If in this last case the thermometers be exposed to simple radiant heat, assuming the universality of the law, that the absorptive is proportional to the radiating power of a surface, we shall have p = k, and p, k, Or hence we might derive a neat and simple method of verifying that law. The relative values of p and k as compared with a surface of glass in particular cases, may be obtained by coating only half the surface of the bulb, and exposing either the plain or k coated side to the same intensity of heat. The ratio may k, be obtained by heating two bulbs completely coated to the same point, and observing their rates of cooling. The case (C) is the same as that investigated in the Phil. Trans. 1800, No. 19, note p. 447. (29.) Comparison of the Focal Effect of two Lenses m a Thermometer coated with Indian Ink. In order to proceed to this comparison, we must first ob serve, that in the formula we take a . d2, it is on the supposi tion that parallel rays impinge on a spherical surface. With the focus this is not the case, and from the convergence of the rays, as well as from their greater intensity at the outer edge, we may in this case assume, without fear of error, that a = the area of the section of the rays impinging, and thus apply the formula. Thus we have the following data: From the sbove experiments, r, = 10, r = 31; it is also evident, that p = p, and kk. By measurement, the diameters of the focal disks were: Lens 1. 0.25 inch .. the areas 0.049 = a d = 0.45. s = .636. α.587, Lens 2. 0.16 inch and we have to apply the case of the formula (B) h 31 587 21 160 1 X = nearly. (30.) In order to calculate the respective intensities of light, or number of rays collected in the focus of each lens, we may easily proceed by the well-known theorem, Let d diameter of aperture, f focal length of lens, No. 1. = And d', f', those of lens, No. 2. I and I'= the respective intensities of the rays collected in the sun's image, of focal luminous disk. Then we have, to By measurement I found In Lens, No. 1, d= 3.25 in... d2 = 10.56, In Lens, No. 2, d' = 1·75 in. .•. d'2 = 3.06, If we admit the validity of certain experiments which seem prove the existence of an exterior heat surrounding the luminous cone of rays, this would affect the bulb in each case by a small quantity in addition to the direct effect of the light. But since the total effect has been shown to be very closely in proportion to the intensity of focal light, it would follow that this exterior heat must be in extent, or in energy, exactly in the same proportion, supposing its absolute value sufficiently great to produce a perceptible effect. These experiments prove for the two particular intensities under examination, that the proportion of heating to illuminating intensity is closely maintained. It might be satisfactory to extend the comparison with lenses of other powers, qualities, &c., but as the above result it not of a nature which requires the admission of any new principles, and agrees with what we should be prepared to expect, I conceive it unnecessary at present to carry the examination any further. (31.) With a similar object in view in some subsequent experiments, I employed such a difference of intensity as is af forded by two sections of the luminous cone formed by a lens, one being made near the lens, and the other near the focus. In two such positions, one, at inch from the lens, the other near the focal point, or at about seven inches distance, the thermometer, blackened as before, was placed successively. The rise in 30 seconds was (mean of three trials), Atinch from lens Near focus 2° = r 40 = 1, To obtain the proportions of light in the two cases, I measured the diameter of the luminous circle formed by the larger section when the rays were intercepted by a plane at the distance of one quarter of an inch below the lens. The diameter was very nearly 2.8 inches d, whose square 7.84; the diameter of the bulb (as before) = 0.45 inch; the diameter of the section near the focus = d = 0.3 inch; its square .09. = = In order to obtain the true ratio of the heating effects, we have to apply the case of the formula (B). By experiment, we have = 1 20 : by measurement a,=.0706. Here also the case of the formula (C) applies, and we have =2; thus on the whole since p = p,, and k=k, 8-a a Hence we have for the intensities of light in the two cases, .09 1 d2 7.84 87 = |