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

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 to 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 troublesome 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

perature. The third column shows the quantity, in grains, that would be evaporated in one minute, if the air had no moisture in it; and the fourth column shows the temperature of the air corresponding to this degree of evaporation, when the evaporating fluid is not supplied by extraneous heat,

A few examples will show the application of this table. 1. If the temperature of the air be 64.5°, then opposite 64.5 in the fourth column we find 114 grains for the evaporation in a minute when the air is dry; but if it has been ascertained that the dew point is at 37°, look opposite 37° in the fourth column, and the evaporation corresponding to that temperature is 5 grains. Take the difference between these, that is, 114-5-64 grains per minute will be the evaporation from a superficial foot when the temperature is 64 and the dew point 37°.

If the grains evaporated in a minute be divided by 600, the result will very nearly express the inches in depth evaporated in an hour, in a still atmosphere.

2. In an atmosphere where the dew point is at 42.3 and the temperature 70.5°, required the depth of water evaporated per hour? This will be the difference between 13.8 and 5'8, or 8 grains per minute. Now is 0134 inches per hour, or a little more than 3 tenths of an inch in 24 hours.

3. A forcing house contains 4000 cubic feet of air, and it is desired to know the quantity of water that would saturate it, the temperature being 70°.

Opposite the temperature 70° in the first column, the moisture which combines with a cubic foot of air at 70° is 7.8 grains, and 7.8 × 4000 is 31200 grains.

4. If the ventilation of a house be 300 cubic feet per minute, and the dew-point of the air admitted be 32° required the surface of water that would maintain the dew-point of the air in the house at 50 when its temperature is 70°?

When the dew-point is at 32° we find each foot of air contains 2-3 grains, and at 50° it contains 4 grains, the difference has therefore to be added by evaporation, but 4-2.3 is 1.7 grains, and 300 x 1.7 is 510 grains. The evaporation from each foot at 70° is nearly 13.8 grains, and at 50° about the mean between 6.8 and 7·8, or 7.3 grains, therefore 13.8—7.3 is 6.5 grains from each foot of surface; and as we have found that 510 grains will be required in that time, we have the quantity by dividing 510 by 6.5, which is very nearly 80 feet of surface.

By this time the reader will have felt that an important inquiry is but slightly entered into, it requires a more complete table, and experiments on the evaporation from moist earth, leaves, &c. to render it more useful. It is not to artificial atmospheres alone that the investigation applies, it may be extended to the face of the globe, and may enable us to trace the effects of cultivation, of stagnant waters in confined districts, and the proper distribution of wood, cultivated land, and water which will preserve a healthy element. Men, as well as plants, feel the exhausting influence of dry air, or perish under the effect of a cold and saturated atmosphere, and perhaps a warm and saturated one is equally noxious.

Those who are acquainted with the researches of Mr. Dalton, on evaporation, will find that his experimental analogy is abandoned, and the subject referred to those first principles which must be involved in the question; and I trust I have done sufficient to show the basis of an accurate theory.

Mr. Dalton's analogy gives nearly true results in low temperatures, but in high ones it is very erroneous; besides not accounting for the well known depression of temperature which must take place where the heat is not supplied from an artificial source.