Thus, by new methods of search, we reach a result which was long ago enunciated on other grounds. Water is well known to be one of the most opaque of liquids to the waves of obscure heat. But if the relation of liquids to their vapors be that here shadowed forth, if in both cases the molecule asserts itself to be the dominant factor, then the dispersion of the water of our seas and rivers, as invisible aqueous vapor in our atmosphere, does not annul the action of the molecules on solar and terrestrial heat. Both are profoundly modified by this constituent; but as aqueous vapor is transparent, which, as before explained, means pervious to the luminous rays, and as the emission from the sun abounds in such rays, while from the earth's emission they are wholly absent, the vapor screen offers a far greater hinderance to the outflow of heat from the earth toward space than to the inflow from the sun toward the earth. The elevation of our planet's temperature is therefore a direct consequence of the existence of aqueous vapor in our air. Flimsy as that garment may appear, were it removed terrestrial life would probably perish through the consequent refrigeration. I have thus endeavored to give some account of a recent incursion into that ultra-sensible world mentioned at the outset of this paper. Invited by my publishers, with whom I have now worked in harmony for a period of twenty years, to send some contribution to the first number of their new Magazine, I could not refuse them this proof of my good will. J. TYNDALL Alp Lusgen, September 4, 1882 * * * * * The German empire has now about 34,000,000 acres of forest, valued at $400,000,000, and appropriates $500,000 even year to increase and maintain the growth of trees. * * * * * APPARATUS FOR MEASURING ELECTRICITY AT THE UPPER SCHOOL OF TELEGRAPHY. _Electro Tuning Forks and their Uses._--On a former occasion I described an instrument to which, in 1873, I gave the name _Electro-Tuning Fork_, and which is nothing else than a tuning fork whose motion is kept up electrically in such a way as to last indefinitely, provided that the elements of the pile are renewed gradually, and that from time to time the metallic contact is changed, which causes, at every oscillation, the current to pass from the pile into the magnet, which keeps up the vibration. We reproduce herewith, in Fig. 1, a cut showing in projection one of the simplest forms of the apparatus. [Illustration: FIG. 1.--CONSTANT VIBRATOR.] If we imagine the platinum or steel style, s, of the figure to be done away with, as well as the platinized plate, I, and its communication with the negative pole of the pile, P, we shall have the ordinary instrument kept in operation electrically by the aid of the electro-magnet, E, the style, s, the interrupting plate, I, and the pile.