from returning, and the same operation being repeated, the water may be raised to any height, not exceeding the above limit, in any quantity. The quantity of water discharged in a given time is determined by considering, that at each stroke of the piston a quantity is discharged equal to a cylinder whose base is the area of a cross section of the body of the pump, and height the play of its piston. The piston, throughout its ascent, has to overcome a resistance equal to the weight of a column of water, having the same base as the area of the piston; and a height equal to the height of the water in the body of the pump, above the water in the well.

4

The lifting-pump. This pump, like the suction-pump, has two valves and a piston, both opening upwards; but the valve in the cylinder instead of being placed at the bottom of the cylinder, is placed in the body of it, and at the height where the water is intended to be delivered. The bottom of the pump is thrust into the well a considerable way, and if the piston be supposed to be at the bottom, it is plain that, as its valve opens upward, there will be no obstruction to the water rising in the cylinder to the height which it is in the well, for by the principles of Hydrostatics,5 water will always endeavour to come to a level. Now when the piston is drawn up, the valve in it will shut, and the water in the cylinder will be lifted up; the valve in the barrel will be opened, and the water will pass through it, and cannot return, as the valve opens upwards;-another shake of the piston repeats the same process, and in this way the water is raised from the well; but the height to which it may be raised is not in this, as in the suction-pump, limited to 32 or 33 feet. To ascertain the force necessary to work this pump, we are to consider that the piston lifts a column of water, whose base is the area of the piston,

and whose height is the distance between the level of the water in the well, and the spout at which the water is delivered.

In the forcing-pump the piston has no valve, but there is a valve at the bottom of the cylinder as seen at A. Opening into the side of the cylinder is another tube, which is bent upwards to the height H, at which point the water is to be delivered; and in this tube, just above the level of A, is a second valve, B, opening upwards. When the piston is raised, the valve in the bottom of the pump opens, and a vacuum being produced, the water is pressed up into the pump on the principle of the suck-bas ing-pump. But when the piston is pressed down, the valve A at the

d

A

bottom shuts, and the valve B at the side which leads into the ejection-pipe opens, and the water is forced up the tube. When the piston is raised again, the valve B shuts, and the valve A opens. The same process is repeated, and the water is thrown out at every descent of the piston; the discharge therefore is not constant.

Popular Encyclopædia.

1. CTESEBES, or Ctesibius, celebrated for his mechanical invention, lived in Alexandria in the reigns of Ptolemy Philadelphus, and Ptolemy Euergetes, about B.C. 250.

2. CYLINDER, a solid, or hollow, roller-like body of uniform diameter, whose ends are equal parallel circles.

3. PISTON, a short solid cylinder, fitting and moving up and down by means of piston rod, within a large, hollow cylinder.

4. SECTION, a division, the plan of an object cut through, as it were, to show its interior. (Lat. seco, to cut.)

5. HYDROSTATICS, the science relating to the statics or equilibrium of water. (Gr. hydor, water.)

METEOROLOGICAL INSTRUMENTS, AND
WHAT THEY TELL US.

The

THE appearance of a barometer1 is familiar to most persons, but although the instrument is very commonly used, at sea, as well as on land, comparatively few are able to understand much about what it tells us. general opinion is, that the rise of level of the mercury usually shows that there will be less wind or rain; its fall that more wind or rain may be expected; that when the level remains steadily high, a long spell of dry weather is probable; while when the level is low the weather will be wet and unsettled, and a sudden change inay be looked for.

These conclusions are correct in many cases, but they require modification in many others, for there are other matters besides mere barometrical indications to be taken into consideration, so that in some instances the barometer taken by itself is wholly misleading. To aid in forming a judgment as to probable weather, three instruments are essential—the barometer, the thermometer,2 and the hygrometer,3 and we shall give a short explanation as to their nature and object.

Briefly we may say that,

(1.) The barometer shows the pressure of the air. (2.) The thermometer (in the shade) shows the temperature of the air.

(3.) The hygrometer shows the degree of moisture present in the air or its dampness.

The Barometer, in its simplest form, consists of a glass tube closed at one end, which is filled with pure mercury, and is a little less than three feet long. It is placed, standing upright, with the open end downwards, in a

cup or cistern partly filled also with mercury. If this be done carefully, without allowing any air to get into the tube, the level of the mercury in the tube will sink until it stands at a height of about thirty inches above the surface of the mercury in the cistern.

The space in the tube above the top of the column of mercury is then empty, forming what is called a

vacuum.

In general terms it may be said that the level of the mercury in the tube rises when the air becomes heavier, falls when it becomes lighter, and remains at rest when it is unchanged in weight.

Atmospherical Pressure. Air like all other substances has weight, and the atmosphere presses on everything at the surface of the earth, with a force or weight of nearly fifteen pounds on a square inch of surface. We do not feel this as a burden upon us, because the tissues of our bodies allow the air to permeate through them, and so the air in them supports the pressure of that outside them. Accordingly, we see that the air presses on the surface of the mercury, in the cistern of the barometer, with a force of about fifteen pounds, on the square inch, and consequently it will keep up such a column of mercury in the glass barometer tube, standing in that cistern, as will press on the same surface with an equal downward force. Now, a cubic inch of mercury weighs about half a pound, so that thirty cubic inches weigh about fifteen pounds. Accordingly a column of mercury thirty inches in height, in the tube, will press on the surface of the mercury in the cistern with a force of about fifteen pounds on the square inch, or with the same force as the air outside the tube does. Every change which occurs in atmospherical pressure will be shown by the rising or falling of the mercury in the tube. If the liquid in the

barometer were water instead of mercury, the column required to balance the pressure of the atmosphere would be thirteen and a half times higher than the column of mercury, because mercury is thirteen and a half times heavier than water. Water barometers have been made in this way, but they are not so handy or useful as mercurial barometers. However, in making an ordinary pump, use is made of the fact that the pressure of the atmosphere will support a column of water in an empty tube the lower end of which is plunged in water. The sucker draws the air out of the pipe, and the water is forced up out of the well after it by the pressure of the atmosphere on the surface of the water round about the pipe, so that in a good pump, which "holds its charge," the water would, if required, stand at a level of more than thirty feet above the surface of the well below.

Effect of elevation above sea-level. It must be remembered that it is only at the level of the sea that the column of mercury in the barometer stands on the average at the height of 30 inches. If the instrument be placed on the top of a hill there will be a lesser thickness of air above it, and if at the bottom of a mine a greater, than there is at sea-level; so in the former case the average height of the column will be less, and in the latter greater than 30 inches. It is most important to bear this in mind, for the difference in the height of the column amounts to more than a tenth of an inch for each hundred feet of elevation above the sea or depression below it.

Motion of the Barometer. The barometer is commonly said to be falling when the level of the mercury in the tube is sinking (at which time its surface is frequently slightly concave or hollow), or when the hand of a wheel barometer or aneroid 5 moves to the left. On the