The Hypochaeris radicata, Hieracium Pilosella, and several others of this family, shut their flowers about three o'clock in the after

noon.

The Four o'Clock Flower is also well known, and is nearly as regular as a watch.

The Pimpernel, Anagallis arvensis, does not open its flowers in the morning when Rain is coming, and has become thereby an indicator of the ensuing weather.

Hence there seems to be some particular periodical influence exerted on certain plants in the course of the day, and on others casually, in particular weather. In general I have remarked that the syngenesious and composite flowers are most under the influence of the former. What this influence consists in is unknown; neither has any conjecture been made, unless that of the electric state of the air varying at stated periods of the day; but the phaenomenon should be more attended to; and the question I beg to submit to future observation is :--whether any connexion can be found between the times of these vegetable periods discovered in the phaenomena of plants, and those periods of the recurrence of the paroxysms of ephemeral diseases observed

by pathologists in the animal system when disordered?

This subject might possibly admit of further illustration, from regular tables of the times of the phaenomena, both of Plants and Animals, and of collateral journals of the electrical changes of the air, noticed by means of the Atmospherical Electroscopes, and of M. De Luc's Column.

There are many other facts, which it would be useless to detail, that illustrate the proposition, that there are other peculiarities of atmosphere, besides heat, cold, damp, &c. which affect the functions of organized bodies.*

* It cannot, I think, be considered, that atmospheric peculiarities alone produce epidemic and other complaints, which must be regarded as having a compound origin, and as resulting from the operation of peculiar states of atmosphere on persons of particular states of constitution; otherwise, all persons would be affected, which is contrary to experience. There are, probably, innumerable varieties of temperament, of general habits of life, and of preexisting diseases, which, in different subjects, vary the effects of the air. And many persons, perhaps, enjoy a state of health, and perfect action, which may be capable of resisting its evil influence altogether. It would, perhaps, be productive of useful results, if physicians of extensive practice would make accurate meteorological registers, during the prevalence of any epidemic or contagious

disorders: such as the influenza, which, a few years ago, took a range for some miles round London, but was also prevalent in other parts of the country.

Since writing the above, I have met with some curious observations on the influence of climates, (which correspond, in some measure, with what I have advanced,) in a French work entitled Rapports du Physique et du Moral de l'Homme, par P. J. G. Cabanis, 2d edit. Paris, 1805. And I have collected a great index of reference to writers on Periodical and other Atmospheric Diseases, in my Observations on the Periodical and Casual Influence of the Atmosphere, &c. &c. London, 1817. And in the Inquiry into the Cause and Mitigation of Pestilential Fever, &c. 8vo. London, 1818.

CHAPTER VI.

SOME PARTICULARS CONCERNING WINDS.*

WIND has been explained in the following manner. Heated air has a tendency to rise, and cold air rushes in to supply its place. Thus the heated air of the equatorial regions rises, and gives place to a current from the polar regions, which is a process that serves to equalize the temperature of the world. But the polar countries lying nearer to the axis of the sphere, the air from those regions has not received so much motion as that about the equator, or greatest distance from the axis; wherefore it arrives at the equator, where the motion of the Earth is greater. If it had no motion before, an East Wind would be the consequence, and the force of that Wind would be as the difference between the motion of the Earth where the air came from, and that where it arrived but then it has a motion to the South; for it is rushing into a vacuum, left by the air which rises: so that the Wind will not

* Tables of the comparative number of days in which each Wind blows on an average, may be found in M. Howard's Climate of London, vol. ii. p. 155.

be from East, but North East; and the number of degrees North of the East from which it will blow will depend upon the comparative force of the current of air from the North to the difference between the Earth's motion at the equator and at the polar region, from whence the air comes. As there must be a corresponding efflux from the equator higher up; according to this theory, the Wind should every where be North East or South West; but it blows in very different directions at different times and places; and this probably depends on the variations in temperature at different times and places.

To partial rarifactions we must ascribe the gales which blow in mountain valleys. I noticed, in traversing the Jura, July 29, 1822, that the Valley Gales were much stronger about half way up, than at the highest parts of the Mountain.

I shall not enter into the detail of the subject, but refer to several treatises written on Winds by different authors.*

* Since the publication of the first edition of this work, I have made many experiments with Balloons, and have observed them always to move in two or more currents, whenever the Wind was not so great as to carry them soon away sight.

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