which the deeper seated cells are filled with mercury; magnified in the same degree.

Fig. 6. The bronchial ramifications traced to their termination at the air cells in the hare; magnified in the same degree.

Fig. 7. A similar portion of the bronchial ramifications in the sheep; equally magnified.

XXI. Theory of the Diurnal Variation of the Magnetic Needle, illustrated by experiments. By S. H. CHRISTIE, Esq., M. A. F.R. S.

Read June 14 and June 21, 1827.

In a Paper published in the Transactions for 1823, I stated

my opinion, that the diurnal variation of the needle was probably due to the influence of temperature, but that the principle adopted by CANTON would not account for the morning easterly variation. In a subsequent paper,* I pointed out that the changes in direction and intensity appeared always to have a reference to the position of the sun with regard to the magnetic meridian; the direction of the needle being undisturbed nearly at the time the sun passed that meridian; and the horizontal intensity being the least at the same time. Having taken this view of the subject previously to my being aware of Dr. SEEBECK's discovery, that magnetical phænomena will arise from a disturbance in the equilibrium of temperature, my knowledge of that discovery and of subsequent experiments, particularly those of Professor CUMMING, confirmed me in the opinion, that temperature must have a considerable effect in producing some of the phænomena of terrestrial magnetism, although I considered that this influence might be modified by the effects produced by rotation, or by peculiar influence in the sun's rays.

At the conclusion of the Papert describing the experiments * Philosophical Transactions, 1825.

+ It appears that this Paper was read before the Cambridge Philosophical

to which I have referred, Professor CUMMING makes this remark: " Magnetism, and that to a considerable extent, it appears, is excited by the unequal distribution of heat amongst metallic, and possibly amongst other bodies. Is it improbable that the diurnal variation of the needle, which follows the course of the sun, and therefore seems to depend upon heat, may result from the metals and other substances which compose the surface of the earth, being unequally heated, and consequently suffering a change in their magnetic influence?" And in the second part of a paper detailing some thermomagnetical experiments, read before the Royal Society of Edinburgh, Dr. TRAILL considers, "that the disturbance of the equilibrium of the temperature of our planet, by the continual action of the sun's rays on its intertropical regions, and of the polar ices, must convert the earth into a vast thermomagnetic apparatus:" and "that the disturbance of the equilibrium of temperature, even in stony strata, may elicit some degree of magnetism."* I am not, however, aware that any thermo-magnetical experiments bearing directly on the subject of the phænomena of terrestrial magnetism, have yet been published.

By varying the original experiment, Professor CUMMING,

Society in April, 1823, but was not published for a considerable time afterwards; and I was not aware of its having been read when I sent my second Paper to the Royal Society in 1824.

• This interesting communication appears to have been read in February, 1824, but I believe it has not yet appeared in the Edinburgh Philosophical Transactions; and I only know it from a short abstract in the Edinburgh Philosophical Journal for October, 1824, with which, however, I was not acquainted, until I had made nearly the whole of the thermo-magnetic experiments described in the present Paper.