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PUBLISHED AT THE ACADEMY HOUSE, 19, DAWSON-STREET.

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LIBRARY

XX.

THE ABSORPTION OF HEAT IN THE SOLAR ATMOSPHERE. BY W. E. WILSON, M.R.I.A., AND A. A. RAMBAUT, M.A., D.Sc.

[Read MAY 9, 1892.]

ONE of the most interesting questions in Solar Physics which awaits solution is-Does the quantity of heat received by the earth from the sun vary from year to year, or is it a constant? Assuming that the internal temperature of the sun remains constant, there are yet two factors, variations in which would cause the amount of solar heat received by the earth to vary. These are the absorption of heat in the solar atmosphere, and the absorption in the earth's atmosphere. It is with the first of these that we propose to deal in this Paper. From the accompanying diagram (fig. 1) it will be seen that the amount of absorption at the edge of the disc of the sun must be much larger than at the centre. This fact has long been known as a matter of observation. Professor Henry of Princeton, in 1845, was the first to discover this, and it has since been confirmed by Secchi, Langley, and many others.

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These observers all make the amount of heat coming from a point on the edge of the disc about half that coming from the centre. It is also an interesting subject of inquiry, how much of the sun's total heat is absorbed by his atmosphere. Laplace, making certain assumptions,

R.I.A. PROC., SER. III., VOL. II.

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found that the sun would be 12 times as bright if he was stripped of his atmosphere. Pickering, taking other data, says that the sun without his atmosphere would be 43 times as bright as it is. Langley and some others have investigated this difficult problem, and whatever the true amount may be, one thing is quite certain that the sun's radiation is stopped to a considerable extent by his atmosphere. If the solar atmosphere varies in depth it is plain from the diagram that the ratio of the absorptions at the centre and edge of the disc will vary, and if accurate measures were taken from time to time of these quantities, we could determine whether the depth of the solar atmosphere was constant or variable. It is with reference to the solution of this problem that we bring the following researches to your notice.

In 1884 Mr. Wilson had an apparatus made to carry out some experiments in this direction. It consisted of two small thermo-piles which were coupled up in such a way that as long as the corresponding faces of both continued at the same temperature the galvanometer remained at zero. At first, both piles were placed on the edge of the solar image formed by a large Cassagrain reflecting telescope. When the galvanometer was steady at zero one of the piles was moved into the centre of the disc. The deflection of the galvanometer was noted. The experiment was then repeated again and again, each time reversing the order of the piles. These experiments gave 0-52 as the heat from the limb, that from the centre being 1.00. Mr. Wilson soon came to the conclusion that some more accurate means would have to be devised before any final result could be reached. In 1888 Professor C. V. Boys invented his radio-micrometer. It is an instrument of extraordinary sensibility to radiant heat, and it occurred to Mr. Wilson that it would be an excellent instrument to use in these researches. In the first place it is so sensitive that we can use an enormously large image of the sun and still get plenty of heat to affect the instrument, and secondly it is very prompt and dead beat in its motion. The sensitive surface on which the heat is allowed to fall is only about 2 m.m. square, so that a very small portion of the solar surface can be examined at one time. Using an image of the sun of 80 c.m. in diameter, the instrument only covers the oth part of the entire disc.

In 1888 Mr. Wilson fitted up a heliostat with silvered mirrors which reflected a small beam of sunlight into a dark room. It was received by a concave mirror of 10 feet focus, and a small convex mirror was placed inside of the focus: this formed a fine image of the sun 80 c.m. in diameter. In the plane of this image the radio-micrometer was set on a heavy slate shelf. A slice of limelight was allowed to fall on the

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