TABLE OF DRYNESS AND WINDS. January 41.0 30.8 35.9 0.3 982 33.0 25.0 29.0 0.0 1000 41.0 33.6 37.3 0.6 983 April. 711 May July August 46.0 33.0 39.5 1.1 963 59.1 43.8 51.4 3.6 874 48.7 34.0 41.3 77.6 56.0 66.8 9.1 798 62.0 48.0 55.0 0.0 1000|| Means. 59.1 43.7 51.4 4.2 877 60-4 44.2 52.2 4.7 859 59.4 43.2 51.3 6.2 733 54.5 38.0 46.2 6.0 749 191 192 DRYNESS IMPORTANT TO RIPENING. The dryness of the atmosphere, which proves so fatal to plants when in a state of growth, is, when accompanied by warmth, of the greatest importance to them while ripening their fruit. Together with the high temperature of the soil, it is this which causes so great a difference in the quality of the same kinds of fruit ripened in the South and the North. The excellence of Syrian Apricots is not approachable in England. The Grapes of the Mediterranean shore are only equalled in England in the best managed hothouses, when sun heat and artificial heat are skilfully employed to dry as well as warm the air, at the season of ripening. The richest and strongest wines in the world are those of Hungary, which, according to Wahlenberg, owe their excellence to the great dryness of the autumnal climate of the valley of the Theiss. Dryness of the air then, which is fatal to plants in a rapid state of growth, is in the highest degree beneficial when their functions are limited to the consolidation of tissues already formed and the elaboration of their final secretions. In the open air in England, the ripening process is usually incomplete, and hence the inability of plants from the United States, and other countries with hot autumns, to bear with us a winter far less severe than that which is natural to them. Nothing can illustrate this truth in a more striking manner than the following statement by the late Sir Augustus Foster. Writing from Genoa he says:·- Being under the impression that single Orange or Lemon-trees, or rows and groups of Orange or Lemon-trees, might with care be brought to grow out of the ground in England like other plants, I have thought it might be worth while to mention the success which has now for several years attended a plantation that I made of seven Orange-trees in a much colder climate, in the garden of my country residence, on the hill of Turin, facing the highest range of the Alps. I was led to make the experiment from having by accident, in the first year of my arrival at Turin, seen the way in which the Orange-trees in boxes were treated in the cellars of a Piedmontese nobleman's house during winter, where they were placed for several months, without light, or heat, or water, and exposed to severe cold which almost every winter reaches to -12° or even-16° of REAUMUR's thermometer (+5° to 4° FAHR.). My group of Orange-trees were taken out of boxes, and planted in earth prepared for the purpose, in the year 1826. In the very severe winter of 1828-9, three of them HARDINESS OF THE ORANGE-TREE. 193 perished, but not of the cold so much as the damp, for they were examined, and seen to be still safe in February, after the frost had reached above 15° of REAUMUR (—2° FAHR.), and perished a few days later from a return of the cold, attended by the drippings of a previous thaw. I had the three which died replaced, and from that time to this they have flourished and increased in size. I have them covered with a round cabin of planks, roofed with straw on the outside, at the end of October or beginning of November, and uncovered in April. They bear abundance of Oranges and Lemons, the former occasionally becoming eatable with sugar. At no other place in this country am I aware that the experiment has been tried, unprotected by a wall. But with a wall and a covering of wood and straw, to be taken off in the summer, I can scarcely doubt that the plants might be made to grow, without the clumsy accompaniment of large wooden boxes, in an English garden." This case establishes the fact that in the north of Italy the Orange-tree bears a degree of winter cold unknown in England. For this it is prepared by the complete ripeness of its wood, a state to which it can never arrive in this climate in the open air. But are we therefore to infer that it will not live with less shelter than it now receives? Such an inference is scarcely justified, and it is worth the consideration of those who have Orange-trees at command, whether they will not pass the winter in barns, or dry out-houses, or under wooden screens where no artificial heating is applicable. Dryness in such an experiment is the first condition to secure; darkness is the second. The Orange-tree will bear to be deprived of water during the whole of its season of rest, provided its roots are kept in the earth they grew in; how much dryness, beyond this, they will bear, is shown by the long exposure to the air which they undergo in the shops of the Italian warehousemen in London; and experience tells us that the effect of cold upon plants is feeble in direct proportion to their dryness. All trees kept in the dark, or at least kept where no sun can shine upon them, will bear without injury a degree of cold.which would be fatal to them if exposed, when frozen, to the direct rays of the sun. Camellias, Chinese Azaleas, Indian Rhododendrons, and many New Holland plants, take no harm in cold pits in winter, provided those pits face the north. Some of them live out of doors perfectly well during winter, if under north walls; and we have in our possession a small Orange-tree which passed the winter of 1853-4, when the thermometer fell to 4° Fahr. uninjured in a cold pit facing the north. As to temperature in the open air, unconnected with atmospherical humidity, there seems to be no means of regulating or modifying it to any considerable extent. In some respects, however, we have even this powerful agent under our control; but in order to exercise such control, it is necessary to understand correctly the theory of what is called RADIATION. This cannot be better explained than in the words of Daniell. "The power of emitting heat in straight lines in every direction, independently of contact, may be regarded as a property common to all matter; but differing in degree in different kinds of matter. Co-existing with it, in the same degrees, may be regarded the power of absorbing heat so emitted from other bodies. Polished metals and the fibres of vegetables may be considered as placed at the two extremities of the scale upon which these properties in different substances may be measured. If a body be so situated that it may receive just as much radiant heat as itself projects, its temperature remains the same; if the surrounding bodies emit heat of greater intensity than the same body, its temperature rises, till the quantity which it receives exactly balances its expenditure, at which point it again becomes stationary; and if the power of radiation be exerted under circumstances which prevent a return, the temperature of the body declines. Thus, if a thermometer be placed in the focus of a concave metallic mirror, and turned towards any clear portion of the sky, at any period of the day, it will fall many degrees below the temperature of another thermometer placed near it, out of the mirror; the power of radiation is exerted in both thermometers, but to the first all return of radiant heat is cut off, while the other receives as much from the surrounding bodies, as itself projects. This interchange amongst bodies takes place in transparent media as well as in vacuo; but in the former case, the effect is modified by the equalising power of the medium. Any portion of the surface of the globe which is fully turned towards the sun receives more radiant heat than it projects, and becomes heated; but when, by the revolution of the axis, this portion is turned from the source of heat, the radiation into space still continues, and, being uncompensated, the temperature declines. In consequence of the different degrees in which different bodies possess this power of radiation, two contiguous portions of the system of the earth will become of different temperatures; and, if on a clear night we place a thermometer upon a grass-plat, and another upon a gravel walk or the bare soil, we shall find the temperature of the former many degrees below that of the latter. The fibrous texture of the grass is favourable to the emission of the heat, but the dense surface of the gravel seems to retain and fix it. But this unequal effect will only be perceived when the atmosphere is unclouded, and a free passage is open into space; for even a light mist will arrest the radiant matter in its course, and return as much to the radiating body as it emits. The intervention of more substantial obstacles will of course equally prevent the result, and the balance of temperature will not be disturbed in any substance which is not placed in the clear aspect of the sky. A portion of a grass-plat under the protection of a tree or hedge, will generally be found, on a clear night, to be eight or ten degrees warmer than surrounding unsheltered parts; and it is well known to gardeners that less dew and frost are to be found in such situations, than in those which are wholly exposed." (Hort. Trans., vi. 8). This very important subject has received further explanation from a writer, whose words we quote, with some omissions, from the Gardener's Chronicle of 1853, pp. 579 and 627. The action of the sun upon all things that receive his rays is a matter of common notoriety. How important to the growth of plants, to the formation of colour and taste, to the ripening of fruit, to the consolidation of all vegetable tissues, is solar light it is needless to say. But few persons are aware of the amount of that force, or of the views of modern philosophers as to the manner in which it takes effect. We may view the surface of a lake exposed to the sun's rays during a warm summer's day, whilst the whole scene may seem to be one of the utmost tranquillity, so that we might naturally conclude that no movement of any importance was then going on. It will be found, however, that such in reality is not the case; for the rays of the sun exert a force of which we can scarcely form any adequate idea. Supposing the lake is only two miles square, it may be calculated that there will be raised from its surface in one day more than sixty-four thousand tons weight of water (64,821), by means of solar radiation. This is at least equal to the work of 10 steam-engines of 200 horse-power each for the same space of time, presuming that the above weight is only raised to an average height of between 300 and 400 feet. To balance that weight, a hill of earth would be required, 30 feet high, 100 feet wide, and 600 feet in length. In making the calculations which have led to these statements, it has been assumed that, in a hot day in summer, a quarter of an inch of water would be evaporated from an exposed surface of a lake in twelve hours, and this from an area of two miles square would amount to 2,323,200 cubic feet, which, at 62 pounds per cubic foot, is equal to 64,821 tons. Now, a quarter of an inch is not a maximum amount of evaporation. The Comte de Gasparin observed 0·59 inch (Gardener's Chronicle, 1849, p. 757), and on five successive days the average exceeded half an inch. Howard, in his Climate of London, has recorded as much as 0.39 inch in one day. It therefore appears that 0.25 inch, that which we have assumed, is not an exaggerated quantity; on the contrary, it is but one-half of that which, according to good authorities, has been actually removed by evaporation, and under a temperature of from 73° to 75° Fahr. Instead of 64,000 tons, facts would justify us in stating that 130,000 tons might be raised in one day from a surface of water not exceeding two miles square. Some idea may be formed from these statements of the immense |