communicated to Lord Melville, when Treasurer of the Navy, in 1792: whether the lifeboat were borrowed from it is more questionable.

cannon.

Mr. Bremner also takes this opportunity of asserting his and of locks to claim to the invention of applying locks to cannon, which he communicated to the late Sir Charles Douglas so long ago as the year 1768.

III.

On the Scale of the Barometer, and the Construction of an Airpump for procuring a perfect Vacuum. In a Letter from a Correspondent.

SIR,

To Mr. NICHOLSON.

I FEEL much obliged by the insertion of my paper on the Airpump, in your very valuable Journal. Should the following hint, respecting the construction of the barometer, (which is at least new to myself,) appear to be worthy the attention of your readers, it is much at your service.

barometer.

In Mr. Dalton's Meteorological Observations, page 7, Observations of where he is speaking of the barometer, I find the following the scale of the remark: "The scale in strictness ought not to be full "inches, but something less, owing to the rising and falling "of the surface of the reservoir. If the tube have a bulb, "then the area of the surface at the top of the column, di"vided by the sum of the areas of the top and reservoir, "will give the part to be deducted; but if the tube be "straight, then the whole area, of the reservoir, lessened by "the area of the glass annulus, made by a horizontal sec❝tion of the erected tube, must be used as the denomi"nator of the fraction; hence, if the fraction be ', then "the scale of 3 inches must be diminished by half a tenth." At page 9 the following observation occurs. "With re

"spect to the barometers at Kendal and Keswick, they "were both clear of air and moisture, and exhibited the electric light in the dark. The scales were both full inches, and therefore the variations were somewhat greater

"than

1

108

Airpump for a perfect vacuum,

AIRPUMP FOR A PERFECT VACUUM.

p. 225.) It may not be amiss to observe here, that it seems necessary, that a correction for this should be made, in taking observations with the barometer; as the mercurial atmosphere will react upon the surface of the top of the column, and prevent it from rising to the full height, to which it would otherwise attain.

By the following means, a perfect vacuum, I believe, may be obtained. Let A B (fig. 2) De a tube of metal, ground so as to be perfectly cylindrical in the inside: let C C be the piston-rod; and D the piston, which is solid; and let a bea small metallic valve, opening outwards: also let the concave and convex surfaces of the barrel, and of the piston, be ground accurately to each other. Let us imagine the piston to be at the top of the barrel, and that all the air is expelled by means of the valve a: if the piston be now forced downwards, the space above it will be a perfect vacuum; at least with respect to air, and all evaporable fluids. The valve might be placed in the piston, as represented by the dotted lines at b; and in this case the one at a would be unnecessary. The action of this pump might be rendered more secure, by closing the bottom of the barrel, and inserting in it a metallic valve, opening outwards; and by making the piston rod to move in a collar of leathers: the piston also and the bottom of the barrel might be ground to each other. By these means, there would be but little danger of any air forcing itself into the vacuum, which would be perfect above the piston, and nearly, if not quite, so below it. This addition is shown by experiments the dotted lines. If the barrel were made of glass, we should then have it in our power, to observe the appearance of the electric fluid, in a perfect vacuum; which, I believe, has never yet been the case. The bottom of the piston also might be formed of different metals, and exposed to the action of a burning glass, or of a galvanic battery. If any elastic fluid were generated by the process, it would be easy to collect and to ascertain its nature. Many other solid substances might be acted upon, by fixing them accurately into the bottom of the piston, so as to form a part of it; and by using the same agents, as in the former case, The principle upon which this pump is constructed I

in which some

might be made.

consider

!

consider as perfect; its application is by no means so: it may nevertheless be a useful hint to those, who are engaged in very refined experiments on the nature of the metals, &c. It would be less troublesome to construct, and perhaps more Another con struction for

applicable to most purposes, if made after the following the same pur

; plan. Let A B be a glass cylinder, D the piston, and E E a plate of glass, large enough to cover the top of the barrel. When this instrument is to be made use of, let the piston be forced upwards, so as to project about a line or less above the cylinder; let the glass plate E E (which must be well ground to the top of the piston) be laid upon the piston, and let it then be drawn downwards; the plate will be kept in its place, by the pressure of the atmosphere, (or other means may be made use of to keep it more secure) and the vacuum will be perfect, as in the former case. The glass E E may be made of a piece of common plate glass, if truly ground; and the focus of the lens will easily be directed through it, so as to fall upon the bottom of the piston.

I am, Sir,

Your obliged and constant reader,

L. O. C.

pose.

IV.

A Description of a Forcing House for Grapes; with Obser-
vations on the best Method of constructing them for other
Fruits. By T. A. KNIGHT, Esq. F. R. S. &c.*

So much difference of opinion prevails among gardeners Construction

houses for

fruit.

respecting the proper forms of forcing houses, that two are of forcing
rarely constructed quite alike, though intended for the
same purposes; and every gardener is prepared to contend,
that the form he prefers is the best, and to appeal to the
test of successful experiment, in support of his opinion.
And this he is generally enabled in some degree to do,
because plants, when properly supplied with food, and
water, and heat, will succeed in houses, the forms of which
are very defective; and proper attention is not often paid

• Trans, of the Horticultural Soc. Vol. 1, p. 99.

by

Best inclination of the glass.

by the gardener, when his prejudices satisfy him, that his labours cannot be successful. It is, however, sufficiently evident, that, when the same fruit is to be ripened in the same climate and season of the year, one peculiar form must be superior to every other, and that in our climate, where sunshine and natural heat do not abound, that form, which admits the greatest quantity of light through the least breadth of glass, and which affords the greatest regular heat with the least expenditure of fuel, must generally be the best and if the truth of this position be admitted, it will be very easy to prove, that few of our forcing houses are at present ever moderately well constructed. I therefore think, that, if plans and descriptions of such forcing houses, as theory and practice combine to prove to have been properly constructed for the culture of every different species of fruit, were published by the Horticultural Society, much useful information might be conveyed to the practical gar dener: and under these impressions I send the following description of a vinery, in which the most abundant crops of grapes have been perfectly ripened within less time, and with less expenditure of fuel, than I have witnessed in any other instance.

It is well known that the sun operates most powerfully in the forcing house, when its rays fall most perpendicularly on the root: because the quantity of light, that glances off without entering the house, is inversely proportionate to the degree of obliquity, with which it strikes upon the surface of the glass; and it is therefore important to every builder of a forcing house to know by what elevation of the roof, the greatest quantity of light can be made to pass through it. To ascertain this point, I have made many experiments, and the result of them has satisfied me that, in latitude 52, the best elevation is about that of 34 degrees: and relative to that elevation the position of the sun, in different parts of the year, will be nearly as represented in the annexed sketch, pl. IV, fig. 4, which is taken from the vinery I have mentioned. About the middle of May, the elevation of the sun will nearly correspond with that of the asterisk A, and in the beginning of June, and again early in July, it will be vertical at B, and at Midsummer it will at C be only six degrees from

from being vertical. The asterisk D points out its position
at the equinoxes, and E its position in midwinter.

Place of the

vines.

In this building, which is forty feet long, and is heated Flue. by a single fire place, the flue goes entirely round without touching the walls; and in the front a space of two feet is left between the flue and the wall, in the middle of which space the vines, which are trained to the roofs, about eleven inches from the glass, are planted; and as both the wall and flue are placed on arches, the vines are enabled to extend their roots in every direction, while, in the spring, their growth is greatly excited by the heat, which their roots and stems receive from the flue. Air is generally admitted at the ends only, where all the sashes are made to slide, to af- Air admitted at the ends, ford a free passage of air through the house, when necessary, to prevent the grapes becoming mouldy in damp seasons. About four feet of the upper end of every 3d light of the roof is made to lift up, (being attached by hinges to the and by lifting wood-work on the top of the back-wall) to give air in the up some of the lights. event of very hot and calm weather; for I prefer giving air by lifting up the lights, to letting them slide down, because when the former method is adopted, no additional shade is thrown on the plants.

The preceding plan is here particularly recommended for

plicable to a

glass injudici

a vinery only; but I am confident, that, by sinking the The plan apfront wall below the level of the ground, and making a small pine stove. change in the form of the bark-bed, the same elevation of roof may be made equally applicable to the pine stove, and that no upright front glass ought, in any case whatever, to be used; for light can always be more beneficially admitted Upright front by adding to the length of the roof, if that be properly ele- ous. vated; and much expense may be saved both in the building, and in fuel. For forcing the peach or nectarine, I must, however, observe, that I think any house of the pre- not fitted to ceding dimensions wholly improper; and I purpose to sub- the peach or mit a plan for the improved culture of those fruits to the Horticultural Society at a future opportunity.

This house

The vine often bleeds excessively when pruned in an improper season, or when accidentally wounded, and I believe Composition no mode of stopping the flow of the sap is at present known bleeding of to gardeners. I therefore mention the following, which I vines.

discovered

to stop the