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No. II.

Tue term Pneumatics is from a Greek word, and is used to express that particular branch of Science which treats of the mechanical properties of Air, such as its weight, density, compressibility and elasticity. The immense mass of permanently elastic fluid which surrounds the globe we inhabit, must consist of a general assemblage of every kind of Air which can be formed by the various bodies that compose its surface, most of these however, are absorbed by water; a number of them are decomposed by combination with each other; and some of them are seldom disengaged in considerable quantities by the processes of nature. Hence it is that the lower atmosphere consists chiefly of * Oxygen and Nitrogen together with moisture and the occasional vapours or exhalations of bodies.

The atmosphere is about fifty miles high or deep, and presses with a weight of fifteen pounds on every square inch of the surface of the Earth –that is, a column of Atmospheric Air one inch square reaching from the surface of the Earth, to the summit of the Atmosphere, would press that small portion of the earth on which it rested, viz. the one square inch, with a pressure of 15 pounds. All things on the earth are exposed to this pressure; those who have never directed their attention to this subject will perhaps be surprised at this very simple fact, and may perhaps wonder how the human frame is capable of existing under

* Atmospheric Air consists of Nitrogen and Oxygen, 79 parts of the former and 21 of the latter in every 100 measures, or 77 of the former anıl 23 of the latter, in every 100 parts by weight,


such an immense burden ;* we walk about and feel no sort of inconvenience, we are not bowed down by the immense weight which our shoulders support, we can move our hands and arms freely upwards and downwards, backwards and forwards, and how is it that we are enabled to do so ? Simply because the Atmospheric Air presses in all directions, upwards, downwards, and sideways; and it is owing to this "equal pressure that we are not injured by the vast weight which we sustain ; for the equal pressure on all sides resists as much as it is resisted. Whenever the hand is held out in this fluid, we feel no weight upon it, because the pressure under and above it are equal, but if we lay our hand on a hollow cylinder of glass placed on the plate of an air pump, and exhaust the air out of the cylinder, we become immediately conscious of something that presses it so forcibly to the glass that it cannot be removed; the prop or support is now gone, there is no Atmospheric Air under the hand, and a column of air nearly 50 miles high presses the hand and the glass cylinder together.

It is owing to this pressure of the atmosphere that we are enabled to raise water from a well by the common pump, the means of which are easily understood; the water in the well is pressed on every square inch of its surface by a column of air weighing 15 pounds; if then we put down the well a long tube or pipe, inserting its end below the surface of the water, and by means of a fitting apparatus exhaust the air contained within the tube or pipe, it follows that water will rise in the pipe to such a height as will just balance the pressure of the atmosphere on the surface of a column of water in the well of the same size as that contained within the pipe, and which will be about 35 feet. Higher than this, water cannot be raised by the common pump. If it were not for the pressure of the atmosphere constring

* Supposing a man's body to contain about 15 square feet of surface (which is near the truth) he will sustain a body of air upwards of fourteen Tons weight.

ing the vessels in men and vegetables, the elastic fluids contained in the finer vessels would inevitably burst them. Persons ascending in balloons have gone 80 high that in consequence of the diminished pressure of the atmosphere, they have had their hands and feet so swelled that it was necessary for Surgeons to make incisions in the skin; at the height which they had reached, the pressure of the atmosphere was not sufficient to counterbalance the pressure of the fluids of the body.

A cubic foot of atmospheric air at the surface of the earth, weighs about one ounce and a quarter; the same measure of water weighs about 1,000 ounces, so that common air is about 800 times lighter than water. When the air is at rest we can move in it with the utmost facility; nor does it offer to us a sensible resistance, except the motion be quick, or the surface opposed to it considerable, but when that is the case, its resistance is very sensible, as may be easily perceived by the motion of a fan.

I purpose to close my brief remarks on the pressure of the atmosphere, with a short description of the Barometer. This instrument more properly belongs to that branch of science which is comprised under the head of Meteorology, but as its action is the result of the pressure of the atmosphere, I will introduce it here.

This instrument was invented by Torricelli in the following manner; having suspected that the pressure of the atmosphere was the cause of the ascent of water in pumps, and supposing that quicksilver being 14 times as heavy as water, the air could only support a column of it one-fourteenth part the height of water, he took a long glass tube closed at one end, this he filled with quicksilver, and inverting the tube immersed the open end under the surface of quicksilver, in an open vessel ; as he expected the quick, silver in the tube subsided so as to stand only about 27.3 inches above the surface of the quicksilver in the open vessel, which was equal to about one-fourteenth the height of water in pumps, lie therefore inferred that both were kept up by the same cause, and that the difference in heights arose from the difference of weight in the two articles. It was afterwards observed that the height at which the quicksilver stood in the tube was not always the same, and consequently, that the pressure of the atmosphere was variable; and also that this change of height was connected with alterations in the weather: hence the name weather glass. There are several forms of barometer; among the most frequently seen are the upright Barometer, which is but very little altered from the instrument with which Torricelli made his experiment, the other is the wheel Barometer, a somewhat more fashionable and elegant form than the upright, although it is scarcely its equal in merit; both are alike dependant on the atmospheric pressure for their variations. A great variety of observations have heen made by different persons relative to the effect which certain changes of weather have upon this instrument, and thence they have derived a set of rules that assist in en. abling us to foretel the changes in the weather. But these are by no means certain and so much to be depended upon as many suppose. So numerous are the causes that effect the state of the atmosphere, and as a consequence alter its pressure; therefore no instrument can be implicitly depended upon as foretelling the alterations that are to happen in the weather. Did my limits allow I might add some rules for the guidance of those persons who are in the habit of consulting the Barometer, a future opportunity will perhaps occur. The height of the quicksilver in the barometer tube is called the standard altitude, and the difference between the greatest and least altitudes is called the limit or scale of variation, the quicksilver in the barometer tube will subside or rise till the column be equivalent to the weight of atmosphere. In this kingdom the standard altitude fluctuates between 28 and 31 inches, and from hence it is justly infered that the greatest, least, and intermediate weights of the atmosphere upon a given base, are respectively equal to the weights of a column of mercury upon the

same base, whose vertical altitudes are 28 or 31 inches, and some altitude contained between them.

The barometer is used to measure the height of mountains. From what has been already stated, it is evident that the higher we ascend in the atmosphere, the shorter its column above our heads, therefore its pressure will be the less; the quicksilver in the barometer tube being less forcibly pressed by the weight which it has to counterballance, subsides and thus indicates the height we have ascended; it is with this instrument that persons in balloons measure the height which they are above the earth's surface.

Dr. Cotes has demonstrated that if altitudes in the air be taken in arithmetical proportion, the rarity of the air will be in geometrical proportion. Example,

At the altitude of 7 miles above the earth it is 4

times thinner and | lighter than at the » Earth's surface.

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This distinguished missionary laboured with much zeal in Persia, without much apparent success. One circumstance, however, which occurred after his death, proved that the seed sown by him was not always lost. The fol lowing narrative was given by a Persian to an English gentleman, who enquired of him the cause of his being converted to christianity.

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