accordance with the general laws of induction, is proportional to the intensity of the principal current. The excess of heat obtained by this kind of induction is proportional to the square of the intensity of the principal current. On the other hand, the square of the intensity of the principal current is proportional to the mechanical force used in surmounting the resistance which the inducing and induced currents exert on being approached to or removed away from each other. Hence it follows that, when induced currents are produced by approaching or separating the induced and the inducing circuit, heat is produced by induction. In this case the production of heat is proportional to the mechanical force exerted in approaching the two circuits or in separating them.

The author has proved by a mathematical deduction, which cannot be reproduced here, that the variation in the disengagement of heat of the principal current arises from induction-currents of a higher order which are produced by the primary

current.

XLI. On the Diminution of Direct Solar Heat in the Upper Regions of the Atmosphere. By J. M. WILSON, M.A., F.G.S., Fellow of St. John's College, Cambridge, and Mathematical and Natural Science Master of Rugby School.

IN

To the Editors of the Philosophical Magazine and Journal.
GENTLEMEN,

N the March Number of the Philosophical Magazine Professor Tyndall has drawn attention to a note of mine in the previous Number, on an observation of Mr. Glaisher's respecting the readings of the shaded and exposed thermometers at great heights.

Professor Tyndall restates my conclusions, and, not admitting them, however fairly drawn, to be representative of natural facts, suggests a possible source of error in the observations arising from a supposed small capacity of the black-bulb thermometer for absorbing the invisible solar rays. I will not occupy your space by quoting Professor Tyndall's statement, but referring to it would remark that, even granting any amount of transparency to the invisible rays on the part of the black bulb, the phenomenon noticed by Mr. Glaisher cannot be thus accounted for, as far as I see at present, without adopting my conclusions which Professor Tyndall quotes. He is doubtless right in saying that the indications of the black-bulb thermometer are more Phil. Mag. S. 4. Vol. 31. No. 209. April 1866.

T

or less delusive, or that we must be careful not to misinterpret its readings; but the special failure of the black bulb which he suggests as possible would not explain this perplexing pheno

menon..

For if the black bulb were opake to the visible and transparent to the invisible rays, the case would be as follows:-There issue from the sun calorific rays, as is commonly supposed, of various qualities, which we may class as visible and invisible rays. Both fall on our atmosphere and pass through it, suffering loss as they descend, but the invisible rays suffering most, from their greater absorption (as is known) by the aqueous vapour in the atmosphere. Now if the black bulb were opake to, and therefore absorbed, the visible rays, but were transparent to, and therefore, we will suppose, totally reflected the invisible rays, the two thermometers at great heights would differ only by the amount due to the direct action of the visible rays. Now they differ, as Mr. Glaisher assures us, by nil; therefore the effect of the visible rays on the black bulb at great heights, where the visible rays are at a maximum, is nil; and therefore, of course, if they suffer no change in their transit through the atmosphere, their effect at all points in their path down to the surface of the earth is nil. The invisible rays, moreover, as they descend are more and more absorbed, and therefore affect both bulbs equally, and have less and less effect by direct action on the exposed bulb, if by this hypothesis they have any effect at all. Why, then, the two thermometers should differ so much on the earth's surface it is not easy to see. It cannot be in consequence of one of them being exposed to the visible rays, for they produced no effect above; nor in consequence of the invisible rays, on this hypothesis, for, if they affected it at all, they would affect it most where they themselves are greatest, at great heights.

I am obviously not supposing that Professor Tyndall made this hypothesis with a view to explain the phenomenon in question; but it is as well to point out that his remarks, though very valuable as indicating to meteorologists a possible limitation of the powers of the black bulb which may have escaped them hitherto, do not at all set this question at rest. But his hypothesis is somewhat startling, for one imagines glass transparent to brilliant, and opake to obscure heat; and it would be strange if blackening the glass reversed both these properties at

once.

I am aware that the pyrheliometers of Herschel and Pouillet have given grounds from which it has been concluded that if

much heat again from the sun as we actually do receive. And I know too, by personal experience, as does Professor Tyndall, how at great heights one feels sensations commonly attributed to solar radiation; and the conclusions in my last paper, which seemed to contradict these and multitudes of other facts which occur to everyone, surprised me and staggered me, I confess. But the truth is that the contradiction lies between, not the facts, but the received interpretations of the facts; and when this is the case, the interpretations must be reexamined; and the process is one which leads to discoveries.

Whatever hypothesis is offered, the fact remains, that at great elevations the shaded and exposed bulb show very nearly the same very low reading, so low that it may be presumed that they would show the same reading exactly if they could be taken to a sufficiently great height. Mr. Glaisher, I will repeat, reasserted this, in the most explicit manner, as based on very numerous and entirely concordant observations both of the black-bulb thermometer and water-actinometer.

The only explanation that I can suggest that is based on known physical actions is the following.

If the black-bulb thermometer does not indicate the direct radiated heat, it may be that it loses it by instantaneous radiation. It is contrary to our experience that this instantaneous radiation is due to diminished pressure; but it may be due to the nearly total absence of aqueous vapour. It is the presence of the atmosphere, and especially its aqueous vapour, that protects the earth from radiating away its heat rapidly by night. It may be the presence of aqueous vapour round the bulb of the exposed thermometer that prevents it also from radiating away its heat by day, and that prevents our being chilled by radiation, even in sunshine, down to the temperature indicated in the shade, a temperature which, but for the aqueous vapour, would be much lower than it is. Where, then, the exposed bulb is protected from loss by the opake aqueous vapour, it will show the heat it receives; where it is unprotected, it will lose this direct heat as fast as it receives it, and indicate the temperature of the air. Should this be the case, it will be an interesting result of Professor Tyndall's discoveries of the high radiating and absorbing powers of aqueous vapour, that it protects the earth and us from instantaneous radiation (that peculiar sensation we feel at great heights and call burning)—that, in a certain sense, if it does not create heat for us, but for it we should have no heat to lose.

It is quite unnecessary to point out the difficulties in accepting this; but to this we are driven if the observations are correct,

the instruments not entirely fallacious, and heat exists in stellar

space.

Professor Tyndall says, in his 'Heat considered as a Mode of Motion,' p. 389, "The withdrawal of the sun from any region over which the atmosphere is dry must be followed by quick refrigeration. The moon would be rendered entirely uninhabitable by beings like ourselves through the operation of this single cause with an outward radiation, uninterrupted by aqueous vapour, the difference between her monthly maxima and minima must be enormous." If the radiation were instantaneous, Mr. Glaisher's observation indicates that the surface would not be heated; it may not be safe to extend this to rough surfaces, as to sand, as it is conceivable that the reciprocal radiation among the particles may be communicated to the mass; but except on some such insecure ground as this (rejecting, as before, the hypothesis of the non-existence of heat, as such, in stellar space), I see no way for reconciling the above inference of Professor Tyndall, or the similar remarks by Herschel (Outlines of Astronomy, §. 431), with Mr. Glaisher's observation.

I have selected the comparison between the bright and dark halves of the half moon as an example which gives a pointed illustration of the difference between my view and the received opinions: it seems paradoxical; but is it contrary, not only to opinions and to inferences, but to facts? In suggesting it I confess I see less ground for hesitation in the physical difficulty of the view than in the mere fact of differing from two such authorities as I have quoted against myself, from whom I can only differ with a mental reservation that further thought and more study on my part would probably bring me to adopt their conclusions. I trust that Professor Tyndall will continue to give his attention to this interesting subject.

I remain, Gentlemen,

Your obedient Servant,

Rugby, March 10, 1866.

JAMES M. WILSON.

XLII. On Molecular Physics. By Prof. W. A. Norton*. [Continued from vol. xxx. p. 289.]

TERRESTRIAL MAGNETISM.-In accordance with the ideas already advanced as to the essential nature of electrical excitation (vol. xxx. p. 107), we may conceive that the earth may derive its magnetic condition from currents developed in its crust by the impulsive action of the æther of space upon the molecular atmospherest. Both the rotatory and orbital motions of the earth may be concerned in the production of such currents. The rotation of the earth should develope currents at each point of its surface, starting in a direction parallel to the equator, and flowing from east to west. Also, if we consider the points of the earth lying on or near the meridian whose plane passes through the sun, and designate the velocity of the earth in its orbit by V, and that of rotation by v, the absolute velocity of the points in question will be V+v on the side opposite to the sun, and V-v on the side toward the sun. The current in the former case, due to the velocity V+v, will run from east to west; and that in the latter case, due to the velocity V-v, will run from west to east. The intensity of the former may be represented by m(V+v)2, and of the latter by m(V−v)2. Taking the difference between these two expressions, we obtain as the excess of the intensity of the east and west current over the other, 4mVv. Such, then, would be the intensity of the effective current at any point, due to the combination of the velocity of rotation and the velocity in the orbit. At points of the earth's surface at any moment in the vicinity of the meridian at right angles to that just considered, the currents developed, so far as they originate in the tangential action of the æther, will be wholly due to the earth's rotation. At certain distances from this meridian, the component of the orbital velocity, in a direction parallel to the surface, will exceed the velocity of

From Silliman's Journal for January 1866.

† As intimated in a former part of this memoir, the priority in the publication of the general theory that the earth derives its magnetic condition from its collision with the æther of space is conceded to Professor Hinrichs, of the Iowa State University, and formerly of Copenhagen. But the idea was no less an original one with the author; and his conception of the essential nature of dynamic electricity, and the magnetic condition of the earth, and his physical theory of terrestrial magnetic phenomena as resulting from the same supposed original cause, are materially different from the views advanced by Professor Hinrichs. It will be seen also that the theory now presented is but the complement to a previous series of researches upon terrestrial magnetism, prosecuted at intervals through a period of about twenty years, and a natural offshoot from the theory of molecular physics propounded in this paper.