fine weather, with the wind from the same quarter, will continue until the barometer begins to rise or fall, when a change may be looked for. 643. Dr. Kirwan has endeavoured to deduce, from observations of the weather extending over a period of forty-one years, the probable chance of particular seasons being followed by certain other seasons; thus, A season is termed wet when it contains two wet months, or when the quantity of rain which falls exceeds five inches. A dry season is one in which the quantity of rain which falls is less than five inches. Variable seasons are those in which there fall between 30 and 36 pounds of rain. A pound of rain is equal to 157637 of an inch. A dry spring is always followed by a rainy winter. Wet summers are generally followed by severe winters. A moist and cold summer, and mild autumn, are sure signs of a hard and severe winter. A moist autumn, with a mild winter, is generally followed by a cold and dry spring. A severe autumn forebodes a windy winter. A rainy winter predicts a sterile year. A frosty winter is followed by a dry summer. A mild winter is succeeded by a wet summer. Springs and winters, if dry, are cold; but warm, if moist. On the contrary, summers and autumns, if dry, are hot; and the former, if moist, are cold. Dr. Kirwan states, as the result of numerous observations, that, If the last week in February, and the first fortnight of March, be rainy, and attended with frequent appearances of the rain-bow, a wet spring and summer may be expected. When there has not been any storm before or after the vernal equinox, the ensuing summer, five times out of six, is dry. If there be a storm on the 19th, 20th, 21st, or 22nd of March, from the S.W. or W.S.W. the succeeding summer, five times in six, is wet. When a storm arises from an easterly point, on the 19th, 20th, or 21st of March, the succeeding summer, four times in five, is dry. When a storm arises in any point, on the 25th, 26th, or 27th of March, and not before, the succeeding summer, four times out of five, is dry. When it rains plentifully in March, it will rain but little in September, and vice versa. About one year in five is characterised by extreme drought, and one in ten by extreme wet. The latter is uniformly cold, the former uniformly warm. A hot summer, says Humboldt, is not necessarily followed by a cold winter. A windy season can scarcely fail, in some part, to be wet. CHAPTER III. RESPIRATION AND CIRCULATION. BEFORE entering upon the further consideration of our subject, it is desirable we should take a brief survey of respiration and its phenomena, both in the vegetable and animal kingdom. 644. Respiration is common to plants and animals. 645. Respiration is the act of receiving a portion of atmospheric air, or water impregnated with air, into the lungs or respiratory apparatus, by inspiration, whence, after a brief interval of time, it is expelled in the act of expiration. 646. Respiration is effected by the contact of atmospheric air, or of aerated water, with the circulating fluids in their passage through the innumerable capillaries ramifying on the delicate membranes constituting a respiratory surface. 647. In plants this function is performed by the leaves. 648. The products of respiration in the animal and vegetable kingdoms are essentially different. 649. Animals rob the air of its oxygen, and exhale into it carbonic acid gas. 650. Plants absorb from the atmosphere, and decompose, carbonic acid, fix its carbon, and restore the oxygen to the air. 651. This is more particularly the case during the day, and especially during sunshine. 652. During the night, and in the shade, the leaves of plants absorb oxygen, and give off carbonic acid. 653. Professor Burnet has referred these phænomena to respiration and digestion. The former is supposed to be with out intermission, and to be attended by the formation and exhalation of carbonic acid gas; the latter to take place only during their exposure to light, and to consist in the decomposition of the carbonic acid of the atmosphere, the absorption of the carbon, and exhalation of the oxygen. 654. The fungi and the fig-tree offer exceptions to the general rule. Marcet found that mushrooms converted the oxygen of the air into carbonic acid both during the day and night. According to Pepys, the perfectly healthy leaves of the fig-tree evolve oxygen even during the absence of light. The Jerusalem artichoke derives its nitrogen immediately from the atmosphere. 655. The leaves of plants also withdraw water from the atmosphere. 656. The decomposition of carbonic acid by plants is proportionate to the intensity and duration of the light to which they are exposed. The quantity absorbed is directly as the force of vegetation. 657. Oxygen is not evolved by plants except carbonic acid be present in the air. 658. Aquatic plants decompose the carbonic acid of the air in the water, and give off oxygen during the influence of light. Part of the carbonic acid contained in the waters of the ocean, lakes, and rivers, is doubtless derived from the respiration of fishes, and would accumulate to a noxious extent were it not replaced by the oxygen emitted by the plants. Hence, fish never thrive in waters which are exempt from vegetation. 659. Plants, then, are as indispensably necessary to the existence of animals as are animals to the existence of plants, for these inhale that which animals exhale, and exhale that which animals inhale. 660. Further, animals unceasingly produce that which plants 654. Lindley's Botany, 3rd edition, p. 376. Phil. Trans. 1843, p. 329. 656. Calvert et Ferrand, Ann. Ch. et Ph. Août, 1844. |