Hygrometer is an instrument deserving of employment in the stove, green house, and conservatory nearly as much as the thermometer; for the correct degree of dampness of the air is of very great importance in the cultivation of plants, and scarcely less than that of the temperature in which they vegetate.

The perspiration from the leaves of plants increases with the air's dryness, and decreases with its moistness. If it be excessive, not only are their juices too much reduced, but the very texture of the leaves is destroyed. If, on the other hand, the perspiration is prevented, the juices are too watery, and the secretions and assimilations are devoid of consistency, rendering the plants too succulent and weak.

"It is impossible for any one to know what degree of moisture he really maintains in a forcing-house without an instrument by which to measure it: that instrument is the hygrometer, which might as well be called the 'water-gauge,' which is what the first word really means. Of the many contrivances to effect this end, the best for all practical purposes, is Daniell's Hygrometer, of which the annexed cut (Fig. 93) exhibits the general appearance. It measures the moisture in the air quickly and precisely, and is not subject to get out of order.

"If moisture is brought into contact with a substance sufficiently cold, a part of the moisture is condensed, and is so converted from a state of invisible vapour into water.

" Thus, in a cold day, the glass roof of a green-house may be seen streaming with water, which runs down and forms 'drip;' and in this often unsuspected manner air is rendered dry, notwithstanding the operations of syringing, steaming, etc. Daniell's Hygrometer is constructed with reference to this circumstance. The figure represents two hollow glass balls containing ether, and communicating by the glass tube which rests on the support. The ball which forms the termination of the longer leg is of black glass, in order that the formation of dew on its surface may be the more perceptible. It includes the bulb of a delicate thermometer dipping in the ether, its scale being inclosed in the tube above the ball; and whatever change takes place in the temperature of the ether is indicated by this thermometer. The other ball is covered with muslin. In making an observation it is first necessary to note down the temperature of the air ; next turn the instrument, so that when the muslin-covered ball is held in the hand, the ether may escape into the blackened ball ; and it should also be held till the included thermometer rises a few degrees above the temperature of the air, when it should be replaced on the support. Then drop, or gently pour, a little ether on the muslin.

The evaporation of this extremely volatile substance produces cold ; and attention must be instantly directed to the black glass ball and included thermometer. The latter will be seen falling rapidly; and at length a ring of dew will appear at the line which runs across the black ball - quickly, if the air is very moist, slowly, if the air is dry. If the air is very dry, no moisture will be thus deposited till the thermometer falls to, perhaps, 10°, 20°, or 30° below the temperature of the air. But at whatever temperature the dew forms, that temperature should be noted as the dew-point; and the difference between it and the temperature of the air, at the time, is the degree of dryness according to the indications of this hygrometer. Thus, in a moderately dry day, let it be supposed that the temperature of the air is 65° in the shade, and that the muslin requires to be kept moist, before dew is formed, till the blackened ball containing the ether has its temperature reduced to 50°, as indicated by the included thermometer, there are then said to be 15° of dryness.

"Again, supposing the temperature is 85°, and the dew-point found, as before, to be 70°, the degree of dryness is still expressed by 15°; but the quantity of moisture diffused in the air is, notwithstanding, somewhat greater in the latter case than in the former.

"If 1000° represent complete saturation, the quantity of moisture, when the temperature is 65° and the dew-point 50°, will be 609°; but when the temperature is 80° and the dew-point 70°, the moisture will be represented by 623; these numbers being ascertained by tables prepared for the purpose. The difference, however, in such a case is so small it is not worth taking into account in a horticultural point of view. But as these numbers can only be ascertained by calculation it is more convenient to reckon by the degree of dryness, bearing in mind that the dryness of the air is indicated by the difference between the temperature of the air and of the dew-point. Thus, if the ring of dew is formed as soon as ether is applied, and only 1° difference is observable, the air is nearly saturated ; if the difference is 5° to 10°, the dryness is very moderate; while 15° to 20° of difference indicate excessive dryness, and beyond this the air is parching." - Gard. Chron.

Fig. 93.

"The instrument," says Mr. Ross, "should be held so as to obtain a portion of bright reflection where the dew is expected to appear; because the dew is most easily seen where the line divides the bright and black reflections on the bulb; and inasmuch as the change may not be noticed the very instant that it occurs, it is well to make a second observation of the temperature at which the dew clears off, and then take the mean of two. If they are both taken equally late, the errors will balance each other; because in one case the mercury is falling, and in the other rising." - Gard. Chron.

Mr. J. W. Harris, writing on the same subject, says: -

" As I have for the last three months, used an instrument for the purpose of regulating the moisture of the air in my orchidaceous house which has perfectly answered my purpose, I am induced to offer it to your notice. It consists of an old-fashioned instrument commonly sold in the opticians' shops as Leslie's Differential Thermometer. It is arranged so that, when not in use, the fluid stands at zero in the stem ; over the bulb of the opposite stem I place a piece of muslin, which has been well soaked in a strong solution of common salt in water. The muslin having been cut into a circular shape, is laid on the bulb whilst wet; and the moisture will make it adhere sufficiently. A shelf, or bracket, with sides, top, and back, is made for it to stand in, to seclude it from the sunshine - which is of course essential - and also to prevent the damp wall from having effect upon the muslin, so that it may draw all its moisture from the atmosphere alone. It will be found convenient to have a thermometer hung on the same stand, as in all hygrometrie observations the state of the thermometer must be attended to. The rationale of its action is simple.

If the absorption of moisture exceeds the evaporation from the muslin, heat will be generated, which will expand the air in that bulb, and drive the fluid up the opposite stem, indicating the degree by its rise. On the contrary, if the evaporation exceeds the absorption, cold will be produced, causing the fluid to fall. The general range of the scales made is from zero to 40°. I believe, in my stove, under the general treatment of orchidaceous plants, temperature ranging from 78° to 95°, the hygrometer has ranged from 15° to 303. of course, if the instrument were found to require it, it would be lengthened in the stem, so as to range to any degree required; but I do not anticipate that a greater range would be required for the coldest pit or green-house. As I have found it very useful in my own stove, I hope it may be of service to your readers; and as it is self-acting, so I trust it will be found on trial, ' simple, economical, and effectual.'" - Gard. Chron.