altitude. In all I have tried I found them agree pretty well down to 28 inches: below this they get irregular and stop at about 27 inches. The moment I have notice of the shipment of the instruments I shall let you know.

We remain, yours faithfully,

JOHN ADIE,

ALEX. ADIE & Son.

Edinburgh, 19th July 1850.

Dr. BUIST, Secretary to the Bombay Geographical Society, Bombay.

In compliance with my intimation in May, when the account of the experiments on the Aneroid in the Annual Report of the Society was laid before you, of trying a series of experiments with the Aneroid at altitudes exceeding 1000 feet, the level at which it had been formerly tried, I took three instruments with me to Poona in the end of July. Two of these belonged to Mr Treacher, one to the Society, this last being sadly out of order: the cause of error was not observed at the time-it arose from the slackening of some of the screws, since tightened.

The results were the following-the Standard Barometers employed were Nos. 1 and 2, two of the finest sent out by Mr Adie-the way they kept together was quite admirable. The Mountain Sympiesometer referred to was a very elegant instrument procured for Colonel Campbell, whose indications were also very accurate, and in most perfect harmony with those of the other instruments.

On comparing the instruments at Sewree, about seventy feet above the level of the sea, they stood on the 22nd July at 10 A. M., as under, the thermometer being 84°, the correction for temperature of the barometer here applied 149-the Standard at the Observatory at this date was 29 667, the instrument being thirty-two feet above the level of the sea.

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The following were the readings of the instruments respectively at Poona at 10 A., M. on the 27th-the Observatory Standard had betwixt these two dates sunk from 29.667 to 29.587-or by 00.080:

The coincidences here betwixt the barometer and mountain sympiesometers, and Mr Treacher's aneroids, are as close as may be.

These experiments were performed at Colonel Grant's, at the extreme end of the Artillery lines-his house is pretty nearly on a level with the church, the top of the spire of which is set down in the Trigonometrical Survey at 2038 feet above the level of the sea. were only cut to 27.5

Mr Treacher's instruments

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inches and that belonging to the Society, cut to 23, was unserviceable, so that the doub expressed by Mr Adie as to whether or not aneroids are trustworthy below to 2g inches for survey purposes, remains unsolved. I do not know if there be at the presidency

a good air pump with barometer gauge; but it seems surprising that any difficulty should be
experienced in determining a matter so simple and so important as this, when a few strokes
of the pump would settle it satisfactorily, without the necessity of ascending an elevation
at all.
Mr Dent quotes Professor Lloyd's authority on the subject of an air pump
experiment-his views were most favorable to the aneroid, and altogether opposed to
those of Mr Adie. Mr Glassher, of the Greenwich Observatory, again, holds very lightly
of the value of the instrument. I took our own Aneroid to the top of Bapdieu Ghaut
along with me the following were the results; but, as already stated, the instrument was
unserviceable, so that no conclusion from its indications can in this case be drawn-the
perfection of the Mountain Sympiesometer is very remarkable :-

The barometer is corrected for temperature to 32°.

The Barometer at the Colaba Observatory stood on the 23rd August at 29.845, or 1.893 higher than that at Poona: if to this be added .030 for the difference between 7 A. M., at which the upper instrument, and 10 A. M. at which the lower one, was read, we shall have a difference of 1.923, or, adding 3 for difference of elevation betwixt the Colaba and Sewree standard, almost exactly the same as that originally set down as the result of the first comparison. At Poona the range between the 26th and 31st July was about .040, that at Bombay about 070-that for the latter part of August at Bombay about 090:-I have no note of the Poona range of this date, but assume it at 6, and have taken the half, While in Poona I took with me three Aneroids with a sympiesometer into the carriage, and drove over the station to see with what facility the instruments could be employed in flying surveys. I did this repeatedly. On one occasion I was accompanied by Colonel Grant, on another by Captain Studdert; it is needless to give details-compared with the barometer the coincidences were of course always wonderful: on one occasion we took a series of levelled stations, where the accuracy was surprising. The great recommendation, both in their case and that of the sympiesometer, was the facility with which they could be observed: by pulling up the horses for a couple of minutes the scale could be read and marked at once, and one hundredth of an inch being allowed for ten feet of change of level, which it is at this elevation pretty nearly, no reductions of any sort were requisite these could be performed at home afterwards. It is quite clear that the makers of the instrument at home have generally underrated its value for survey purposes. It is in its present form in many cases unworthy of the high state of instrument making. The scale, for example, ought always to extend round the whole dial plate so as to read as low as twenty-three inches, and to be cut out at the very extremity of the diameter of the plate,-by this means it would be enlarged by nearly a third. Each division corresponding to an inch in the barometer would thus become 18 inches in length, and each tenth of an inch might be divided into tent hs, instead of quarters as at present, Troughton often cuts the scale of his ma. rine barometers into hundredths of an inch, so as to be read without a vernier-each of the divisions recommended would be four-fifths on those referred to; still a vernier might very

easily be added if requisite. The brass register on the glass should be omitted, as likely only to endanger the glass itself; and nothing can be more absurd than to encumber the dial, and shorten the divisions of the scale, by putting round the border the words stormy, much rain, rainy, change, fair, in the case of an instrument meant for something else than an old woman's weather-glass. At Poona, at an elevation of 2000 feet, it fluctuates betwixt 27.8 and 28.5, so as to be, according to the legend, in a state of perpetual tempest even at the finest season. There are a multitude of alterations in the mechanism of the instrument which could easily be improved-those I have adverted to are so obvious that every man who procures instruments for use in India ought to insist on them.

DR. FORD'S ATMOMETER.

This most beautiful instrument was exhibited before the society with the exception of the glass tube and thermometer, it had been entirely manufactured by the inventor at Hoshungabad, where no assistance of any kind could be obtained. In point of finish it was scarcely inferior to the workmanship of a London optician. The knife-edge fulcra of the balance rested in gems beautifully let in on the top of the pillars. The tongue of the balance projecting downwards, was fitted with a little sphere of brass, which screwed up and down to alter at pleasure the position of the centre of gravity, and the extremity of the tongue moved against a finely cut arch beautifully graduated. The whole as a piece of art, independent of the merit of the invention, was most perfect, and excited the admiration of all the members present.

DESCRIPTION OF AN ATMOMETER, BY DR. FORD.

This Atmometer consists of a Standard, with a double lyriform head, carrying two polish ed garnet tables, on which rest knife-edges of a skeleton beam. The needle of the beam points below to a graduated index. At the ends of the beam are two steel stirrups, riding on knife edges. To one stirrup is hooked a long hollow glass cylinder, capable of holding full three cubic inches of water. At the lower end of the cylinder is attached by a cap a large male screw, perforated for the admission of water into the tube. Within the cappiece is a conical valve, which opens when the screw is fitted into the female screw at the side of the evaporating vase; a hole in the side of the latter, corresponds to the perforation in the male screw, and makes thus a communication between the contents of the vase and of the glass cylinder, and ensures, as from a fountain, a supply of water to the vase, propor tioned to the evaporation.-At the side of the vase the bulb of a capillary thermometer just penetrates the water when the vase is filled, and indicates the temperature of the eva porating surface. The diameter of the vase is 2.78 inches, affording an extremely small excess over six inches area. This excess may be considered annihilated by the immersion of the thermometer bulb, leaving strictly six square inches for evaporating surface. Upon the screw piece of the vase, is attached a flat plate, whereon to place the weights employ. ed to ascertain the loss of evaporation. And to arrive sooner at the knowledge of the probable weight that will be required, the glass cylinder has been scaled to inches and one twentieth of cubic inches, and 1-40th of an inch can be readily estimated. A glance, therefore, at the loss in the cylinder will point out the number of grains, within a grain or so, needed to restore the equilibrium.-In the upper metalic cup of the cylinder there is a milled-head screw fitting air-tight, and is removable for the purpose of cleaning the cylinder at any time. The parts that will come in contact with water are made of white metal, as being less liable to corrosion from its action than brass or copper. The other

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stirrup carries a stout brass wire, to which the counterpoise is hooked. This exactly balances the beam when the cylinder and vase (with Thermometer attached) are filled with distilled water. There is a contrivance fixed to the upper and back part of the standard actuated by a screw, by which the knife edges can be raised from the planes; and the beam has two notches on its lower surface, which are received upon two teeth on the arms of the lever, so that, when liberated, the beam can always be brought to the same part of the planes. The whole stands on a mahogany board with three levelling screws, a spirit level at the back, and weights from to 550 grains in a row in front. The beam, though light sustains 12,000 grains, and will then indicate the addition of a of a grain—or the 48,000th part of the load. It is as exact as can well be, the difference between the two arms not amounting to one 20th of a grain. I regret that I had not another thermometer to spare for attachment to the standard, as the temperature of the air must be registered conjointly with that of evaporating surface.

The directions for using this instrument are very simple. The vase is filled up to the upper part of the opening in it, and the cylinder is unscrewed from the vase, and by reversing it the valve falls back, the water is then poured in, and the valve, upon inversion, comes again into play and prevents the escape of the water. It is then screwed into the vase. Should the whole now weigh more than counterbalances the given equipoise, a small quantity of the water can be removed by the corner of a handkerchief, and the true balance adjusted; or, if found wanting, a few drops can be added. To make certain that no air exists at the mouth of the valve, and intercepts the connection between the water in the cylinder and that in the vase, it is as well to tilt up the cylinder margin of the vase, and pass up the bubble of air, if one be present, or the same end may be effected by a straw or wire passed through the opening in the vase into the hollow male screw. This will liberate the bubble equally well, although with careful manipulation no air should lodge at the point of junction,

After the equilibrium has been obtained, either the entire apparatus may be raised by the gear referred to, or the cylinder or the equipoise weight, or both, may be detached from the beam. The whole should be placed under a large cage made of fine bamboo sticks, with large interspaces, otherwise birds and insects will give an erroneous value to the evaporation. This circumstance is overlooked when the ordinary evaporating dishes are employed.

I was induced to devise, and construct, the above described instrument, in the hope that by its use the solution of one of the most difficult problems of Meteorology might be facili. tated, if not overcome. By it all theoretical views as to the force of vapour at various temperatures are avoided-the units here, without a risk of extraneous errors, being the grain weight, the cubic inch, the degree of Fahrenheit's Thermometer, and a given interval of time. The Barometer will of course be a most valuable adjunct. As weighing, however, is a far more accurate process than measuring, it should invariably be had recourse to in preference, and the obtained weight subsequently be reduced to cubic inches if required. Even in graduated narrow tubes it is difficult to fill up so that the measure shall at once correspond with a given weight. Let any one fill a wineglass to a certain mark, and note its weight; fill it, to all appearance, as before, and then remark the disparity when it has been weighed. Success in these attempts amounts to a sheer impossibility when a vessel of large area is employed. It is this circumstance that renders the common evaporating dish so untrustworthy. It is, likewise, most essential that the evaporating surface should be maintained at one mean height, otherwise, when it is on the gradual decrease, and falls much beneath the margin of the vessel containing the water, the evaporation i retarded. This desideratum is fulfilled in the present instrument, since the fountain will act only as required.

At present every formula (be it Dalton's, Ure's, Robinson's, Galbraith's, Joory's, or of others—and they all differ from each other) employed to ascertain the amount of evaporation, is more or less empirical and arbitrary. In this Atmometer, authorized units are only used, and so long as the accuracy of the balance remains unimpaired, and any degree of attention be paid to the registrations, the results must approach in character and degree those of a natural process.

As no strictly true value can be allowed to the ever varying power and dryness of the wind, it will be more advisable to place the instrument in a shaded open space, cut off equally from irregular currents of air and refracted heat.

Hoshungabad, July 16th, 1850.

C. G. E. FORD.

Two years ago I had the pleasure of directing the attention of the Society to the extreme difficulty of carrying out hygrometrical experiments,-pointing out the extraordinary degree of neglect and inattention with which the subject of evaporation had been treated. The common wet and dry bulb, the hygrometer of Daniell, and others, merely indicate the quantity of moisture in the air-evaporation is not only dependent on this, but on the velocity of the wind, I then laid before you a model of an atmometer on the steel-yard principle, consisting of a balance, with unequal arms,-the shorter of these having an arched segment at the extremity. From this an evaporating dish, like that of Dr. Ford, was suspended; this, when filled with water, was so adjusted and counterpoised as to be in equilibrio with the steel-yard when this stood in a horizontal position, and of course as the water was drawn off by evaporation, the long arm descended, the amount of descent