it off? Can I kill it with any sort of wash? Will it come again? Does it do much harm? Such questions as these, though often answered, are becoming so common that it seems desirable to bring them all under one reply at this time of the year, when whatever has to be done should be done quickly.

"No explanation of the manner in which what is called 'moss' can be dealt with is worth having, unless the reason why it comes is clearly understood: we will therefore at once say that moss or lichen grows on the bark of trees for three reasons:-1. The bark is dead; 2. The bark remains without change; 3. The air around it is loaded with moisture.

"It is an invariable law of nature that the surface of a tree dies after a time; this may be seen even in young branches, the skin of which has become brown, for brownness is among plants an absolute indication of death, ferns and some seaweeds excepted. As the branch grows older, superficial death becomes more evident, the bark cracking or crumbling away. In this state it is, physiologically speaking, mere humus or mould, upon which anything will grow, the seeds of which can attach themselves to it and subsist upon the elements of the atmosphere. Lichens fulfil such conditions; their invisible seeds, floating in the air, fall on trees and hold on them by the mere force of attraction, sprout there, incorporate themselves with the bark, and at last grow up into visible plants. Hence all trees may be attacked by lichens, mosses, and similar plants.

"But all trees are not attacked, or at least we see no sign of their being so. This arises from the rapid decay, or the frequent sloughing, of the surface of certain trees; the consequence of which is that the ground, so to say, on which the inosses stand is perpetually slipping from under such incumbrances and carrying them away. The ceaseless expansion of the wood brings this about. This year a branch is an inch in diameter, or 3 inches round; next year it is 14 inch in diameter, or 3 inches round. The original surface must therefore occupy inch more than before; but, having been already fully formed, it has undergone no additional growth, nor will it have stretched beyond a very small amount; it merely gives way beneath the internal distension, cracks, peels off, or crumbles away. Under such circumstances, the seeds of any lichens that may have attached themselves to the bark must also fall away without having disclosed themselves. It is when the bark undergoes change very slowly, if at all, that the lichens have time to establish themselves, and to form the shaggy beards that load some orchard-trees. This absence of a sufficient expanding force is owing to the tree having ceased to grow with sufficient vigour: to grow is to form wood; to grow vigorously is to form wood abundantly; to form wood abundantly is to lose surface rapidly; to grow fast is therefore to render the presence of lichens impossible. But we may reverse the description: to grow slowly is to form wood slowly; to form wood slowly is to lose the power of casting off the surface-bark; to grow slowly is therefore in the highest degree favourable to the presence of ' moss.'

"Now, trees grow slowly because either of ill health or extreme old age. The latter is without cure; for the former there is a remedy. Young trees (from 7 to 75 years old) get into bad health from the soil in which they grow being too poor, or too wet, or in some other way unsuited to their constitution. If too poor, manure is a remedy; but this is seldom the case: men rarely plant orchards in sand or peat. It will almost always be found that the fault of the soil is its wetness and its impermeability. The first is attended by a low temperature, and morever gorges the system of the tree with water, which it can neither throw off as perspiration nor decompose; the second cuts off the requisite supply of atmospheric air, stops rain when the roots most want it, as at fruiting time, and renders it impracticable for the roots to wander freely in search of food. For all this the remedy is deep digging and deeper draining. A moss-grown orchard trenched two spades deep, and drained four feet,

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will, in any soil fit for fruit trees, rapidly recover its health and keep it. Under such circumstances wood will form fast, bark will peel off, or crack, or crumble away, and, with the surface, the lichens themselves will also go. At the same time the health of the tree will be restored, and good fruit will follow. But it is not merely because draining relieves the soil of water when injurious, and brings it when the roots most want it, that it is indispensable to the health of orchard-trees; it also acts by rendering the air that rests upon the branches itself too dry to suit the constitution of the lichen. Bark may be dead, and may remain in its place for years, and nevertheless the lichens cannot grow unless they find themselves surrounded by a moist atmosphere. Getting no nutriment, or very little, from the bark itself, they of necessity feed upon whatever the air contains, and on nothing more greedily than on the water there, without which all other agents are useless.

"It is needless to point out the enormous difference in the amount of water floating in air which rests upon undrained and thoroughly drained land.

"Such being the true history of 'moss' on fruit-trees, it is obvious that scraping, and washing, and painting, can have no permanent value, if any. When spending time upon it, it is assumed that the moss is itself injurious. We do not believe that it does any harm whatever. It is merely a symptom of decrepitude, removeable or irremoveable, as the case may be, and a natural warning to gardeners that their trees require better treatment.

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"It may, indeed, like rifts in bark, harbour insects, and in such a way be injurious; but this is a small matter, and easily remedied by a scraper. ill, however, be found that, as soon as the improved soil begins to act, no further care beyond skilful pruning and thinning is demanded."

VII. - On the Practice of Irrigation. By HENRY TANNER, M.R.A.C., Professor of Agriculture and Rural Economy, Queen's College, Birmingham.

THE important effects which have been realised from the practice of irrigation naturally lead us to look at the causes producing them; and although the principles upon which successful irrigation depends are by no means fully or satisfactorily understood, yet I shall endeavour to explain them, so far as they can at present be looked upon with confidence. The results which render water-meadows valuable are of two classes: an early production of food and an increase of quantity. In noticing the causes of these effects, we shall not find any conditions but such as are of general application to vegetable growth. The present inquiry will show that they are solely referable to the more complete influence of the same principles which control all agricultural produce. Early production is chiefly a question of climate. Naturally, this is shown in different districts, in some of which the mild spring weather, being earlier than in other parts, leads to an earlier commencement in the growth. The cold of winter produces a torpidity in the circulation of all vegetation, and from this hybernal slumber it is aroused by the stimulating influence of spring. As the temperature increases, so we find this torpor yields to the energy of life; hence the function of vegetation recommences, and a further growth is the result. Now there are various modes of producing an artificial climate, and by their adoption we can, to a certain degree, imitate, and thus anticipate, natural seasons. Amongst the various modes of producing an artificial climate, we must place irrigation. Daily observation proves that by its aid we can obtain the growth of spring during the winter months, and I shall now proceed to explain the principles upon which this practice is based.

The surface-soil has two sources of heat: that which arises from internal heat, and that which is thrown upon it by the solar rays. Now, as warmth is necessary for stimulating into activity the dormant energies of vegetation, it may be inferred that, in proportion as we increase warmth, so shall we produce a more luxuriant growth. Facts prove that such is the case. During the months of winter, it is certain that the heat received from the sun is of little importance as a stimulant to vegetation; it is not intended by the laws of nature that it should act at this season of the year, and therefore no assistance can be anticipated from this source. The atmosphere during the winter months, being at a low temperature, offers increased facilities for the warmth of the surface being drawn off and scattered. Experiments have shown that this heat, in leaving the soil, darts upwards in a straight line, and for this reason it may be presumed that if we could intercept the heat as it is passing away a higher degree of temperature would be maintained.

We are all familiar with the various modes adopted in gardens for applying this principle, but it was applied more extensively some years ago by Mr. Gurney. He had observed * that " if a bush or any other fibrous matter were left lying in a field of grass, the vegetation beneath it would soon be finer and fresher than that around it. This fact was generally known, and had been noticed from time immemorial; but the agency by which this increase of growth was brought about evidently involving some great and important but unknown principle-had never been fully investigated." It was generally attributed to the shelter from the weather, or the protection from cattle, which the bush afforded. This led to a series of experiments being carried out, and Mr. Gurney discovered that if rods of wood, iron, or any other material were supported within an inch of the surface, an increased growth resulted, and the greater the width of the covering material the more evident was the effect. Flags, rushes, straw, bushes, or any similar covering produced the same effect.

Reeds or wheaten straw applied over the grass, at the rate of about a load or a load and a half per acre, in a very short time increased the quantity of grass to an incredible extent. The various grasses under these coverings were found to be healthy, and rapidly passed through their stages of growth: growing, some flowering, some seeding. Mr. Gurney also gives the following experiment :

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An increased number of experiments tended to confirm the preceding, but they added this important fact that the increase of growth was in the same ratio as the varying degree of fertility in the soil. Thus Dr. Vacy says*

"On some coarse moors the increase of growth was very slow, as compared with other soils. It was found that the rate of action was also influenced by artificial manuring, and that the increase of vegetation was in a ratio with the natural quantity that would be produced by a given manure when laid on a field and not assisted by the operation of any fibrous coverings. A certain quantity of stall dung, which would double the quantity of grass when laid on in the usual way, was found to increase it six times when properly treated with fibrous covering. I made a careful analysis of the herbage produced by this action, and also that of the same ground left open, and the results were the same. The fattening proportions seem to be equal, weight for weight. They were tried on feeding-cattle, milch cows, and store stock."

The mode of applying this to practice is exceedingly simple. A ton, or rather more, of straw, rushes, &c., is spread evenly on the grass, and raked up into heaps at the end of two or three weeks-in fact, as soon as the grass begins growing above the covering; and the produce is either consumed, or the covering again applied after one or two weeks' delay. There is no doubt but the value of this process chiefly arises from the warmth of the soil being increased by the protecting agency of the covering of straw.

Another peculiar circumstance connected with these experiments is, that a portion of grass-land covered during the day and left uncovered during the night produced a luxuriant herbage; whilst on another portion, which was uncovered by day and covered by night, a sickly and inferior herbage was produced. This, however, I believe, is referable to the fact of the growth being (in the former case) stimulated whilst the action of light

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-although subdued by the covering-could influence the change in the sap, and produce a healthy growth. In the latter case, the period during which the growth of the grass was stimulated being night, the sun could not complete the change in the sap, and the result was a sickly yellowish growth. From other experiments, it was shown that rods both opaque and transparentrods of wood and branches of trees-all produced similar results. Although the practice of fibrous coverings and irrigation are distinct, still the same principles operate, to a certain extent, in both. In like manner as by the use of various coverings we intercept the loss of heat, and thus raise the temperature of the soil, so also does the shield of water, supplied by irrigation, act. It promotes the growth of grass by the warmth retained at the root of the plant. The value of retaining the warmth of the soil will be rendered evident by a reference to Mr. Whitley's paper on the Climate of the South-west of England,* showing the regularity with which grass land maintains a temperature higher than the atmosphere above it. In consequence of this higher temperature, it is clear that grass land must lose much of its heat when posed to the action of the colder atmosphere, and hence the value of protecting it by a shield. It is done by the fibrous coverings, and the same end is gained by the use of water. This is not the only mode in which the temperature of the soil is influenced by irrigation. If the water supplied be of a higher temperature than the air, it not only acts as a covering, but it also imparts some of its own warmth to the soil, and thereby operates more powerfully. River-water is almost always of a higher temperature than the air, whilst springs and brooks are even warmer than the rivers. It therefore follows as a natural consequence, that the greater proportion of water available for irrigation is of some value in this respect, whilst, owing to their greater warmth, many springs are of considerable value. The warmest springs have the deepest sources, their temperature being raised by the internal heat of the globe. In like manner, water from the deepest Artesian wells has been proved by Arago t to be hottest. The temperature of hot springs varies from 209° Fahr.‡ downwards, and we have every variation down to those which approximate closely to the temperature of the air. In the following Table (Bath and West of England Journal, vol. iii. page 141), Mr. Whitley has given the temperature of ordinary spring water, under 300 feet of altitude, in the south-west of England, and to this I have added the temperature of the air.