Specification of the Patent granted to ALEXANDER Adie, of Edinburgh, Optician; for an Improvement on the Air Barometer, which improved Instrument is to be called a Sympiesometer. Dated December 23, 1818.

To O all to whom these presents shall come, &c. Now KNOW YE, that in compliance with the said proviso, I the said Alexander Adie do hereby declare, that the nature of my said invention, and in what manner the same is to be performed, are particularly described and ascertained in manner following; that is to say: The object for which I claim a Patent is an improvement on the air barometer, which I call a Sympiesometer, and my invention consists in employing an elastic fluid or gas different from air, and any liquid except quicksilver, which neither acts upon the gas which it confines, nor is perceptibly acted upon by the air, to the contact of which it is in some measure exposed. Hydrogen gas, azotic gas, or any of the gases not liable to be absorbed by the inclosing fluid, may be used; but I prefer and employ hydrogen gas. The liquid which answers best is VOL. XXXV.-SECOND SERIES. LI any

any unctuous oil, or mixture of unctuous and volatile oils. I prefer and employ almond oil, coloured with anchusa root. The form and construction of the Sympiesometer are as follows: First, it consists of a tube of glass, of any length or diameter, I prefer one about 18 inches long, and 0.7 of an inch diameter inside, terminated above by a bulb, about two inches long, and half an inch in diameter (but this will vary, as the instrument is required to have a greater or lesser range), and having the lower extremity bent upward, and expanding into an oval cistern open at top. The bulb at the upper end is drawn to a slender thread, and at first is left open. To introduce the gas and oil, I fill the bulb and tube with quicksilver, and close the mouth of the cistern with my finger. A communication is then formed between a gasometer, containing the gas to be used, and the slender pipe at the end of the bulb, by means of a flexible tube; and the orifice of the cistern being then opened, the quicksilver flows out till it descends in the tube to the level of the top of the cistern, and the gas enters to supply its place. The slender pipe is then to be sealed hermetrically close to the bulb, by a touch of the flame of a blow-pipe. The tube is now to be inverted, and the mercury poured out of the cistern, allowing the column which occupies the tube to run towards the bulb, to prevent the escape of the gas: the tube being again turned upright, the portion of quicksilver which remains is removed, by pouring some of the oil over it, and heating the gas until, by its expansion, it forces the column of quicksilver which is left at the lower-end of the tube into the cistern; then, holding the tube nearly horizontal, the oil will enter as the gas cools, and the remaining quicksilver may be poured out of the cistern. The gas introduced, changes its bulk, or occupies more or less space,

accord

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according to the pressure of the atmosphere upon surface of the oil in the cistern. The scale for measuring the change in the bulk of the gas occasioned by a change of pressure, is formed experimentally, by placing the instrument in an air-tight glass-case, along with an accurate barometer and thermometer. The glass-case is furnished with a condensing and exhausting syringe, by which any density may be given to the enclosed air, so as to support a column of quicksilver in the barometer of 28, 29, 30, or any other required number of inches. The height of the oil in the tube of the Sympiesometer corresponding to these points, being marked on its scale, and the spaces between being divided into a hundred parts, these parts correspond with hundredths of an inch, on the scale of the mercurial barometer. As the bulk of the gas is altered by any change that takes place in the temperature of the atmosphere, it is necessary to apply a connection on this account. For this purpose, the principal or barometric scale is made to slide upon another scale, which is divided into degrees and parts, so as to represent the changes of bulk in the gas produced by a change of temperature under the same pressure, and corresponding to the degrees of a common thermometer attached to the instrument. This scale is constructed in the same manner as the scale of a common thermometer, by changing the temperature of the bulb, while the pressure is the same, and noting the range of the oil occasioned by it. In using the instrument, observe the temperature of the thermometer, and set the index, which is upon the sliding Sympiesometer scale, opposite to the degree of temperature upon the fixed scale; and then the height of the oil, as indicated on the sliding scale, will be the pressure of the air required. When the height of one place above another is to be measured by the diLI 2 minution

minution of the pressure of the atmosphere, another correction is necessary to insure perfect accuracy in all instruments indicating this change, because the pressure of a column of air, of a given altitude, varies according to its humidity or moisture. Another of my improvements connected with the Sympiesometer, consists in the invention and application of a new hygrometer, which may be either attached to the instrument or used separately. This hygrometer is constructed of the fine internal film or membrane, which lines the hollow tube of the arundo phragmites. A small bag made of this membrane is filled with quicksilver, and attached to the lower end of a thermometer tube, so as to form as it were its bulb. The quicksilver in the bag occupies part of the glasstube, and rises and falls by any change of humidity, which is indicated upon a scale attached to the tube. The lower end of the glass-tube, instead of being merely inserted in the top of the bag, may pass through it; the quicksilver in the bag communicating with that in the tube, by an opening made in the side of the tube. By this means the bag is supported by the glass-tube, and prevented from being injured by any slight accident, and the instrument is less affected by any change of temperature. The hygrometer may be also made in the same manner as Saussure's, by substituting a slip of the membrane already mentioned, in place of the hair employed by that philosopher. In some of my Sympiesometers the scale is divided into parts corresponding to the increase in bulk, which takes place in the gas by the diminished pressure of the atmosphere on ascending a given height, the temperature being 52° of Fahrenheit. This scale is also formed by experiment as follows: the instrument being placed in the glass-case as before described, increases the density of the inclosed air, until it

support

support a column of quicksilver of 31 inches, the temperature being 32°. Mark this point zero; then from the logarithm of 31 súbstract 0.100, and find the corresponding number, which is 30.294; regulate the density of the air to support a column of quicksilver of this length; number this point on the scale 100, and divide the space into a hundred parts, each part will equal the increase of bulk or fall of the oil in the tube, by ascending one fathom, the ordinary correction being allowed for any change of temperature in the atmosphere. In the above manner proceed by subtracting 0.100 from the logarithm last found, and making the points correspond to these densities until the scale is complete. By subtracting the number of fathoms indicated by the Sympiesometer at the under station, from that indicated at the upper station, the difference will be the number of fathoms which the one place is above the other.

In witness whereof, &c.

OBSERVATIONS BY THE Patentee.

My attention was first directed to the improvement of the barometer, with the view of rendering it susceptible of indicating any of those minute changes in the weight of the atmosphere, which might be supposed to arise from the action of the sun and moon. A very sensible instrument was obviously necessary for such a purpose; and I was therefore led to the idea of measuring the pressure of the atmosphere by its effect in compressing a column of common air. Upon constructing an instrument of this kind, however, I found that the air was absorbed by the fluid with which it was inclosed, and that. a good and permanent barometer could not be made upon such a principle till this radical defect was removed. I therefore directed my attention particularly to this object,