showery; but, if the ground be thoroughly drenched with water by the autumnal rains, no further trouble is necessary. Under this mode of management, the plants will remain perfectly green and luxuriant till their blossoms and young seed-vessels are destroyed by frost, and their produce will retain its proper flavour, which is always taken away by mildew. (Hort. Trans., ii. 87.) CHAP. III. OF ATMOSPHERICAL MOISTURE AND TEMPERATURE. * THE Constituent parts of the atmosphere that surrounds us are either the same in different regions, or the differences, if any, are not appreciable by chemical processes. It is far otherwise, as regards temperature and humidity, which are so intimately connected that they cannot be considered apart from each other. From what has been already stated (Book I. Chap. This subject has already been fully treated by Professor Daniell, in his excellent paper "On Climate with regard to Horticulture," published in the Transactions of the Horticultural Society, vol. vi. p. 1. It is impossible for any one to discuss the same topic without profiting largely by this important treatise, which I have very much followed in the present chapter. IV.), it is apparent that of the vital functions of plants none are more important than those of perspiration and evaporation; and that, while a certain amount of loss of their fluid particles is necessary, a great excess or diminution of the loss must be injurious. Although the solar rays appear to be the immediate cause of perspiration, which proceeds in proportion to their intensity (71.), yet this action is necessarily modified by the state of the medium, that is, of the atmosphere, which surrounds them; in proportion to its heat and dryness will their power be augmented, and in proportion to its cold and moisture diminished. The physiological effect of an excessive augmentation of perspiration is to dry up the juices and to destroy the texture of the leaves; on the other hand, an excessive obstruction of that function prevents the decomposition and assimilation of the fluids, and the formation of new organised matter, as well as of the secretions peculiar to a species. A state of the atmosphere, therefore, which is most favourable to the maintenance of the perspiratory action in the most healthy state, is that which it must be the business of a gardener to secure by all the means in his power. Among the hygrometers intended for measuring the quantity of elastic vapour in the atmosphere, the most convenient for use is that invented by Professor Daniell. In this instrument, the K amount of moisture in a given atmosphere is indicated by what is called the dew-point; that is to say, by the point of the thermometric scale at which the cold is sufficient to cause a deposition of dew the amount being calculated by the difference between the natural temperature and an artificial temperature created for the purpose of determining the point at which the elastic vapour of the air is precipitated by cold. "The natural scale of the hygrometer," says Mr. Daniell, "is included between the points of perfect dryness and perfect moisture: the latter, of course, being that state of the atmosphere at which the dew-point coincides with the temperature of the air. The intermediate degrees may be ascertained by dividing the elasticity of vapour at the temperature of the dew-point, by the elasticity at the temperature of the air the quotient will express the proportion of moisture actually existing, to the quantity which would be required for saturation; for, calling the term of saturation 1.000, as the elasticity of vapour at the temperature of the air is to the elasticity of vapour at the temperature of the dew-point, so is the term of saturation to the actual degree of moisture." By means of this and similar contrivances*, we * Other hygrometers have been invented to answer the same end; but, as Mr. Daniell's is that most eligible in this country, I have thought it more convenient to confine my observations to it. are at all times able to ascertain exactly the quantity of water that exists in an elastic state in the air. In this country, the changes of moisture are said to extend from 1.000, or saturation, to 389, or even so low as 120, under a south wall, for a short space of time; "a state of dryness which is certainly not surpassed by an African harmattan,' but one which produces less disastrous consequences, because it is accompanied by a far lower temperature and a weaker solar radiation. The mean degree of moisture of the air near London has been found by Mr. Thompson to be 897, on an average of ten years, while the mean temperature is 50-62*: in other parts of the world it is very different; and the amount of those differences, together with the means of imitating them artificially, constitutes one of the most delicate and difficult parts of the gardener's art. All that relates to this subject, however, to be treated usefully, must be considered in a very special way, and in such detail as can only be expected in a separate work upon the subject. An idea of the difference between the atmospherical moisture of London and that of other parts of the world may, however, be collected from the following table showing the amount of rain that falls in a few different countries. See the various meteorological journals published by the Horticultural Society, in their Transactions, from the year 1826 inclusive. Bengal, 20 to 22 inches in a single month. Bombay, 32 inches in 12 days. Tavoy, 203.5 inches in six months; as much as We possess, to a certain extent, the power of modifying the moisture of the air even in the open air, and have almost complete control over that of glazed houses. It is found by experience that the effect of wind is to increase the dryness of the air, and, consequently, the perspiration of vegetable surfaces. "Evaporation," says Mr. Daniell, "increases in a prodigiously rapid ratio with the velocity of the wind; and any thing which retards the motion of the latter is very efficacious in diminishing the amount of the former. The same surface which, in a calm state of the air, would exhale 100 parts |