erers, knows the pear, calls it a melting Bonchrétien, a good bearer, and excellent keeper. Mr. Cooke's opinion is to the same effect.

It is amusing to hear of the Standard Napoleon being planted on Coxheath, a spot where, during the war, the flower of the British army were assembled to prevent such a result. Mr. Braddick has purchased a very fine estate in that neighbourhood, which he is improving on an extensive scale, and introducing there, and, wherever he has an opportunity, the most improved varieties of hardy fruits. To this subject he has been devoted for many years, and, perhaps, no man living has originated from seed, or imported from France and America a greater number of excellent sorts. At Boughton, Mr. Braddick is his own architect, agronome, and gardener, and we should be most happy to receive some account of the many operations he has now going forward in building and planting.— COND.

ART. X. On the Cultivation of the Grape known as West's St. Peter, as practised at Spring Grove: By MR. ISAAC OLDACRE, F.H.S., gardener to the Emperor of Russia.

WEST'S St. Peter grape is acknowledged by all who have seen it at Spring Grove to be the finest and best late grape yet cultivated in this country; and although it has been long in England it is but little known amongst horticulturists. I am not acquainted with its having been cultivated in the neighbourhood of London before I planted two vines of it at Spring Grove in the year 1818. It has made such rapid progress in its growth and fruit bearing, that I hope a short history of it in your Gardener's Magazine may be of no small interest to the lovers and cultivators of grapes in general, as, if they adhere to the few hints here given, they may have grapes all the winter months, as plentiful and as fine as at any time in summer.

I begin every year to force this grape in the middle of April, and keep the heat of the house where it grows as near sixtyfive degrees fire heat, by Fahrenheit's thermometer, as I can until the summer months. This grape requires more heat to bring it to maturity than the Hamburgh, or any of the earlier kinds I am acquainted with. The fruit with me begins to change colour in August. When the weather is wet or cold at this season I make a little fire at nights, so as to keep the house at sixty degrees fire heat until the fruit becomes quite black, which is sometimes in the middle, and sometimes in

the end of November, when I reduce it to temperate, and so keep it till I have cut all the grapes. This in some years is the beginning, and some years the end of March. The Poonah grape I keep till April, with the leaves on as if in sum

mer.

The wood of West's St. Peter is round, of a brown colour, short jointed, eyes prominent, leaves rather small, and flat, smooth, and shining underneath, deeply serrated; they turn to a purple colour as the fruit becomes black. The vine grows freely, and is a great bearer; the bunches at first showing are small, and apparently weak, but gradually advance until they become long with large shoulders. The blossom sets freely, the berries are round, and grow of an even size, and if well thinned they soon become large. When ripe, the grape is of a very black colour, the skin thin, with small seeds, very juicy, and high flavoured.

There is another St. Peter grape which is known to most experienced gardeners, but is very different from the one above mentioned; the leaves of this old variety are very downy or woolly underneath, the edges turn downward, the berries are oval, and the wood long-jointed, that is, with great distances between the buds.

Spring Grove, 5th December, 1825.

ART. XI. On the Relations of Heat, Moisture, and Evaporation in Natural and Artificial Atmospheres. By THOMAS TREDGOLD, Esq., Civil Engineer.

THE constitution of the atmosphere has a most important influence on the growth of plants, and particularly its relation to moisture. Till within these few years the variable state of the moisture in the air was not registered with any degree of accuracy; and, chiefly, from the want of proper instruments. A variety of contrivances have been, from time to time, invented for ascertaining the quantity of moisture in the air, but none of them are so perfect as it is desirable to render them. The well-known expansion and contraction of both vegetable and animal substances, by the effect of moisture, has been tried in several ways; but since there is no probability that the change of bulk is exactly proportional to the change in the quantity of moisture, these methods are not in much esteem. The increase and decrease of weight from moisture is objectionable for the same reason.

There are two methods, however, which give accurate results, though they are not quite so easily applied as the principle

of expansion and contraction, and certainly not so well adapted for common observation. The one was first tried by Dr. James Hutton, and consists in observing the temperature as reduced by evaporation. Thus the bulb of a thermometer, being covered with muslin, and moistened with water, it causes the thermometer to sink a number of degrees corresponding to the state of the air in respect to moisture. In principle it is a simple and elegant method, in practice not so precise as the method next to be described.

When air is cooled below a certain temperature, it deposits part of the moisture it contains in the form of dew. Now when air deposits dew it is saturated with moisture, therefore, air always contains that quantity of moisture which would saturate the same quantity of air when its temperature is reduced down to the dewing point or temperature at which it deposits dew. But if a body cooled down to the temperature of the dew point be presented to a mass of air, dew is deposited on its surface; hence it only requires the combination of a thermometer, with a means of reducing the body containing it to such a degree of cold that dew deposits, to have an accurate means of determining the dew point; and, consequently, the quantity of moisture in the air.

A most ingenious instrument has been contrived on this principle by Mr. Daniell, and it has lately been slightly improved by Mr. Jones of Charing Cross. These instruments are, however, rather too delicate and troublesome for ordinary use, and something more simple seems to be desirable. The state of an artificial atmosphere, in regard to moisture may, perhaps, be most satisfactorily obtained by measuring the real quantity of evaporation, it being the excess or the deficiency of this quantity that affects the health of plants; and the evaporation in a given time is nearly proportional to the difference between the quantity that would saturate the air and the quantity it actually contains.

In order that the quantity evaporated in a given time may be sufficiently measurable, a proper surface should be exposed to the air in a cylindrical vessel, having vertical sides (fig. 6.); and

20

a tube of smaller diameter might be added, into which the water may be changed to measure the quantity evaporated. If the diameter of the cylindrical vessel be 1-58 inches, and that of the tube half an inch, then each tenth of an inch in the cylinder will occupy an inch in the tube; and as the mean evapo

ration in our climate will then be equal to about one-fifth of an inch in the tube in twelve hours, this quantity might be made the unit on the scale; and the whole would be evaporated in fifty hours in the mean state of the air.

Place the tube in a perpendicular position, and fill it to zero; then place it with the tube horizontally as in the figure. When it is desired to know the quantity, which has been evaporated, invert the remaining portion from the cup to the tube, and the place of its surface in the tube will indicate the quantity evaporated. The lower part of the cup ought contain as much water as fills the tube to zero.

This instrument will faithfully indicate the power with which the air is abstracting moisture from plants; it is a little trou some in use, but not more so than the hygrometers, which indicate with equal accuracy.

The ordinary state of the atmosphere, with respect to moisture in this country, is extremely variable; but the mean result of many observations, of both the thermometer and the dew point, shows that the temperature of deposition and the actual temperature, follow each other in a regular manner. The difference

between the actual temperature and that of the dew point is least in January, and gradually increases till June, when it again declines to its winter state. Mr. Daniell has observed these phenomena with much attention, and has given the results for three years. The mean of these observations is shown in the small tablet (fig. 7.), where the upper shaded line shows the

mean temperature, and the lower shaded line the mean temperature of the dew point. This species of tabular picture is valuable, because we can so easily observe not only the whole range, but also the range of any particular period. From Mr. Caldcleugh's observations at Villa Rica, the mean evaporation was double that in this country, and the quantity of moisture in the air also nearly double.

It will at once be perceived, that the progress of evaporation must depend on the quantity of water the air is capable of taking up to saturate it, and when it is perfectly saturated, evaporation

must cease.

Also, since a supply of heat competent to form the vapour produced is absolutely necessary to the process, the evaporation must be limited by the supply of heat, and proportional to it. Hence, in the progress of evaporation, there will be cold produced, till the difference between the temperature of the surface affording vapour and that of the surrounding medium be equivalent to produce the proper flow of heat. It must obviously depend on the nature of the conductors which supply it, but must be constant under the same circumstances; and, therefore, the hygrometer of Dr. Hutton is founded on true principles.

But the surface affording vapour being cooled below the general temperature, the quantity of moisture the air is capable of taking up must be estimated from the temperature of the evaporating surface.

In ordinary circumstances, the ultimate depression of temperature is about one degree, by an evaporation of 150 grains per hour, from a surface one foot square, or 2·5 grains per

minute.

It also appears from Mr. Dalton's experiments (Nicholson's Journal, vol. vii. p. 7.) that in a still atmosphere, where the heat is supplied in the quantity necessary to produce the vapour, the evaporation per minute, from a surface one foot in area, is exactly equal to twice the quantity of vapour in a cubic foot of saturated air of the same temperature, or 120 times that quantity in an hour.

From these conditions we have the easy means of forming a table to exhibit the evaporation from an area of one foot of water at any temperature. For this purpose, I will take the weight of vapour which a cubic foot of dry air takes up at different temperatures, when saturated, from my work on Warming and Ventilating, and add the columns for showing the rate of evaporation and temperature of the atmosphere.

The first column shows the temperature of the evaporating fluid at its surface; the second column the weight of water in grains that would saturate a cubic foot of dry air at that tem