I have lately remarked a circumstance with regard to the change of Winds, which I have never heard mentioned by meteorologists, and which may therefore be worth noticing. I have observed, that when the current next the Earth has changed its direction, it has frequently got into and blown from the quarter from which an upper current had previously blown. I was first apprized of this, by observing the motion of an upper stratum of clouds to be different from that of those which were lower; and by the lower clouds afterwards taking the direction of those above: but as I had few opportunities of observing this circumstance, I thought it merely accidental. Subsequent observations on the various directions of Air Balloons, and the succeeding changes of the Wind, have convinced me that it is frequently the case, that the changes of the Winds begin above, and are propagated downwards. And I have observed this of several successive currents.

Persons who are desirous of making these observations, should have Windvanes accurately constructed, and should compare their indications with those of the clouds above. Weathercocks should be made with a ball of oil at the top, so constructed as to keep dropping into the circular cylinder on which the fan

turns round. I had a Vane of this sort constructed, which had a small bell suspended from the point, so that at every change of the Wind I was apprized of it by the ringing of this little Tintinnabulum, as I sat under the trees of the Elm Grove at Walthamstow; and I could, in some instances, hear the sound when in the house at some distance. I contrived this machine in order to ascertain the sort of gales which might blow, as I found them at times blowing straight and steady, but at others so irregular and unsteady, as to produce a constant horizontal vibration of the fan; the consequence was, that the pointed side of the Weathercock corresponding in its motion with it, the little bell kept constantly ringing. I can safely recommend the use of these sort of Vanes, as they are very accurate indicators of the Wind, when constantly lubricated with oil made to drip into them, and they last a long while without wanting repair. The one above alluded to at Walthamstow, was put up in April, 1817, and is still in good order. But they require good workmen to adjust them. I have since tried many times to get a good Weathercock put up at Hartwell, by country mechanics, without

success.

Vanes are of ancient invention, and one of the most perfect was the Aurologium, placed in the garden of Varro; but though so long known, they have never been much improved. At the Exchanges of London, of Lubeck, of Amsterdam, and other great commercial towns, they have indexes in the chamber below where they are fixed, and these indexes are made to move round a face like that of a clock on the wall, the particular Wind being indicated on the dial. The fleche or sagittiform fan is the best shape for Vanes; but almost any preponderance of surface over weight on the side to be moved by the Wind, is enough in moderate breezes to indicate their direction, as we may assure ourselves by observing the cumbersome and whimsical forms of Dragons, Foxes, Griffins, Half Moons, and other capricious devises, which are set up for Weathercocks on steeples and other lofty buildings. When the breeze be very gentle, however, such grotesque Vanes are apt to become useless. The Dragon on Bowchurch Steeple, and the Grasshopper on the Royal Exchange, do not gyrate with those slight movements of the air that impel the Vanes which turn the indexes at Lloyd's, and the Office of the Insurance Company.

CHAPTER VII.

OF ELECTRICITY.

If we look back into the history of any branch of science, we shall observe, that in the progress of its developement, men have, from time to time, introduced a number of different hypotheses to explain the cause of the complicated phaenomena which they observed; which hypotheses have obtained credit for a while, and have reigned triumphant; but before long they have faded away, from being found incapable of explaining more recently discovered facts, or have been overthrown by others of greater pretensions to credit. From time immemorial, systems of philosophy have mutually overthrown and succeeded each other; and many, which have been rejected by philosophers of antiquity, have been brought into vogue again, under some new dress, by subsequent generations: and thus, in the revolutions of science, systems have alternately decayed and flourished at remote distances of time. Electricity affords a striking example of

this. After the two different electric states of bodies, commonly called the positive and the negative charge, were discovered by certain dissimilarity in their effects,* philosophers began to dispute about the state of those bodies. Some contending that when two different electrics were rubbed together, so as to become electrified, the one gained as much as the other lost of a fluid matter, which they called the electric fluid; and that when, by subsequent approximation, or the intervention of conductors, their electric properties ceased, an equilibrium of the fluid in the two bodies was again restored. While others contended for two distinct fluids, which had a sort of attraction for each other. Upon this supposition, the electrification of two different bodies by friction was a separation of the two fluids, one to each electric, and the equalization was a commixture again, or a distribution of both Electricities through both the electrics. Many plausable experiments and arguments were used in favour of each hypothesis: but the former always obtained the

*For example, the difference of appearance of the luminous star on the point of a conductor, when applied to a body positively charged, from that of the star on the poin directed to one negatively charged.