The observations in each line of the table apply to a period of twenty-four hours, beginning at 9 A. M. on the day indicated in the first column. A dash denotes that the result is included in the next following observation.

REMARKS.

Second Month.-1. Clear morning with white frost: fine day. 2. Very fine day. 3. Hoar frost: foggy. 4. Cloudy. 5. Fine lunar halo at night. 6. Ditto. 7. Drizzling. 8. Overcast. 9, 10. Drizzly. 11. Very fine morning. 12. Drizzly. 13. Morning fine: afternoon rainy: night stormy. 14. Cloudy. 15-17. Fine. 18. Cloudy and fine: a little snow in the morning between five and six, which melted immediately. 19. Fine. 20. Drizzly morning: rainy evening. 21. Drizzly: very foggy night. 22. Foggy morning: fine afternoon. 23. Bleak. 24, 25. Fine.

26. Bleak: some hail and snow at three, p. m. 27. Rainy: some hail at half-past eight, a. m. 28. Cloudy rainy evening. 29. Cloudy.

RESULTS.

Winds: N, 4; E, 8; SW, 5; SE, 2; W, 5; NW, 2; NE, 2; Var. 1.

ANNALS

OF

PHILOSOPHY.

MAY, 1824.

ARTICLE I.

Remarks on Solar Light and Heat. By Baden Powell, MA. of Oriel College, Oxford.

(To the Editor of the Annals of Philosophy.)

SIR, April 2, 1824. IN explanation of the design of the present communication, I conceive it necessary to observe in the first instance, that having been engaged in experiments on solar light and heat, I have laid accounts of some of them before the Royal Society (see reports of Royal Society, Annals, Feb. 1824): those accounts, however, being confined to the mere detail of the experiments, I wish through the medium of your journal to offer some remarks on the subject, of a more general nature, and which may be considered as forming a sort of introduction to such experimental researches.

If then in taking a brief review of the present state of our knowledge upon this subject, my remarks and statements should not be of a nature wholly new, my design as thus explained will be a sufficient excuse; and the more so as I could not proceed to the few experiments here given without such preliminary considerations. I am, Sir, your obedient servant, B. POWELL.

I. (1.) Speaking according to our ordinary sensations, we are accustomed to say that the sun communicates both light and New Series, VOL. VII.

Y

heat. Light is transmitted in a way which we term radiation. The heat from non-luminous hot bodies is transmitted to a distance in a way closely analogous; and to which the same name has been applied. In the first instance we might suppose that the sun sends out two separate emanations, one of light, and another distinct from it, and similar to that of radiant heat from a mass of hot water; and this, perhaps, was the first view taken of the subject, though a confused idea of some very close and intimate connexion subsisting between the solar light and heat appears to have prevailed.

(2.) This subject, as might naturally be expected, attracted the early notice of experimenters. A very slight examination sufficed to show that the rays of solar heat (whatever their nature might be) differed essentially in many properties from those of terrestrial heat, whether radiated from luminous or nonluminous bodies. Whether there existed a separate set of heating rays distinct from those of light, and at the same time differing in many respects from rays of terrestrial heat; or whether these differences depended on some unknown property of the rays of light, was a question which for a long time remained without any direct investigation, and on which even now, we have, perhaps, no very precise ideas.

Among the earliest experiments on the subject, if not actually the first, were those of Mr. Boyle, on the different degrees of heat communicated by the sun to black, white, and red-coloured surfaces. These were extended and confirmed in the wellknown investigations of Dr. Franklin, &c.

"Mr. Boyle caused a large block of black marble to be ground into the form of a spherical concave speculum, and found that the sun's rays reflected from it were far from being too powerful for his eyes, as would have been the case had it been of any other colour; and although its size was considerable, yet he could not set a piece of wood on fire with it; whereas a far less speculum of the same form, made out of a more reflecting substance, would presently have made it flame." (Boyle on Colours, &c.)

Scheele conceived that the sun's rays of light produced heat, not when in motion, but only when stopped by the interposition of solid bodies. (Treatise on Air and Fire, &c.)

Mr. Melville seems to have viewed the matter nearly in the same light, and to have conceived reflexion at an opaque surface, the cause of excitation of heat from the sun's rays. (See Phil. Mag. June, 1815, a paper by Dr. Evans.)

(3.) In later times the experiments of Prof. Leslie, Sir H. Davy, &c. have sufficiently established the property possessed by that emanation (whatever its nature may be, whether simple or compound) which is derived from the sun, of producing greater heat in bodies in proportion as their surfaces owing to darkness of colour, have the capacity of absorbing rays of light.

It has been equally well established by Prof. Leslie, Count Rumford, &c. that the heat emanating from a mass of nonluminous hot matter, has no such relation to the colour, though a very close one to the nature and texture of the surface it

(4.) The experiments of Sir E. Home (Phil. Trans. 1821, Part I.) are particularly deserving of attention, as exhibiting what might at first sight be considered an exception to the above remarks; a greater effect being produced in some instances on a white than on a black surface. A more attentive examination, however, will show us that these experiments prove thus much. The heat occasioned by the rays of the sun when received directly, or when in some degree intercepted as by thin white cloth, on the skin, is greater than that communicated by conduction to the same skin, through a black cloth in contact with it, which is itself, in the first instance, heated by absorbing the rays.

A white skin is scorched, and a negro's skin is not, in ten minutes by the direct rays of the sun; that is, as before, the outer coat of the white skin allows some of the direct rays to pass through and affect the sentient substance beneath; whereas in the case of the black skin, the rays are absorbed by the black surface, and so affect the sentient parts only as heat of temperature.

II. (5.) As to the nature of this heating effect, the greatest difference of opinion has long prevailed among the most distinguished philosophers; one party maintaining the totally distinct existence of light and radiant heat in the compound solar beam; the other contending for the absolute identity of the two: the same principle being merely displayed under two different modiǝ fications. (See Sir W. Herschel, Phil. Trans. 1800, Part II.; Leslie on Heat, p. 162, Biot, Traité de Physique, vol. iv. p. 690, &c.)

Without entering upon an examination of the merits of either theory, we may proceed to remark that the first object in the inductive examination of this subject is to ascertain distinctly what peculiar properties of this heating emanation we can fix upon by which its nature may be defined, and by the help of which we may be enabled to compare it with other heating emanations.

(6.) Among the most obvious properties of the solar rays, we perceive that before adverted to, viz. that they produce heat on bodies in proportion to the darkness of their colour, and not in regard to the absorptive qualities of the texture of their surfaces for the heat from non-luminous bodies. This relation is universal, and without any exceptions; it is consequently one which we can satisfactorily adopt as the foundation of a distinctive description.

(7.) We may from this advance to another test, which will afford an additional characteristic. It has been distinctly shown