139. The SIPHON is a bent tube, by means of which a liquid may be transferred from one reservoir to another, over an intermediate elevation. The siphon may be used with advantage when it is required to draw off the upper portion of a liquid without disturbing the lower portion. This operation is called decanting.

The siphon consists of two branches of unequal lengths, as shown in Fig. 103. The shorter one is plunged into the liquid to be decanted, and the flow takes place from the longer one.

To use the siphon, it must first be filled with the liquid. This operation may be effected by applying the mouth to the outer end of

(139.) What is a Siphon? When may it be used with advantage? decanting? Explain the operation. How is the siphon prepared for use?

What is

the siphon, and exhausting the air by suction, or it may be inverted and filled by pouring in the liquid, and stopping both ends, after which it is again inverted, care being taken to open both ends at the same instant. Sometimes a sucking pipe is employed to exhaust the air and fill the siphon.

When the flow commences, it will continue until the liquid in the first reservoir falls below the level of the end of the siphon.

To understand the action of the siphon, we must consider the forces called into play. The water is urged from d towards b, by the pressure of the atmosphere on the fluid in the reservoir, together with the weight of the water in the outer branch of the siphon; that is, by the weight of a column of water whose height is ab. This motion is retarded by the pressure of the atmosphere at b, together with the weight of the fluid in the inner branch; that is, by the weight of a column whose height is cd. The difference of these forces is the weight of a column of the liquid whose height is the excess of ab over cd, and it is by the action of this force that the flow is kept up. The greater this difference the more rapid will be the flow, and the less this difference the slower the liquid will escape. When this difference becomes zero, the flow ceases altogether.

The siphon is used for conveying water over hills, but for this purpose the highest point of the tube should not be more than thirty feet above the level of the water in the reservoir, this being about the height at which the atmospheric pressure will sustain a column of water.

If a siphon be mounted on a piece of cork, so as to sink as the level of the fluid falls, the flow will be constant. Such a siphon is called a siphon of constant flow.

How long will the flow continue? Explain the principle and action of the siphon in detail. How high can water be raised by a siphon? Describe a siphon of constant flow.

IV. APPLICATION то BALLOONING.

Buoyant Effort of the Atmosphere.

140. It has been shown that a body plunged into a liquid is buoyed up by a force equal to the weight of the displaced liquid. That a similar effect is produced upon a body in the atmosphere,

may be shown by means

of an instrument called a baroscope, which is represented in Fig. 104.

The BAROSCOPE consists of a beam like that of a balance, from one extremity of which is suspended a hollow sphere of copper, and from the other extremity a solid sphere of lead. These are made to balance each other in the atmosphere.

If the instrument be placed under the receiver of an air-pump and the air exhausted, the copper sphere will descend. This shows that in the air it was buoyed up by a force. greater than that exerted upon the leaden sphere.

If, now, the leaden sphere be increased by a weight equal to that of a volume of air equal to the bulk of the copper

(140.) What instrument is used to show the buoyant effort of the air? Describe the Baroscope. Explain its use.

sphere diminished by that of the leaden sphere, it will be found, after the air is exhausted, that the balance is in equilibrium. This shows that the buoyant effort is equal to the weight of air displaced. Hence we have the following principle, entirely analogous to the principle of ARCHIMEDES:

When a body is plunged into a gas, it is buoyed up by a force equal to the weight of the displaced gas.

If the buoyant effort is greater than the weight of the body, the latter will rise; if it is less, the body will fall; if the two are equal, the body will float in the atmosphere without either rising or falling.

Smoke, for example, rises, because it is lighter than the air which it displaces. It continues to rise until it reaches a stratum of air where its weight is just equal to that of the displaced air, when it will come to rest and remain suspended. A soap-bubble filled with warm air floats for a considerable time in the atmosphere, being nearly of the same weight as the displaced air.

The Balloon.

141. A BALLOON is a spherical envelope filled with some gas lighter than the air.

Balloons are of very different sizes, and are filled with gases of very different specific gravities, and consequently capable of raising very different weights in ascending to the upper regions of the atmosphere.

The first balloon was constructed by STEPHEN and JOSEPH MONTGOLFIER, two brothers, in 1783. It was made of linen, lined with paper. It was about forty feet in diameter, and weighed 560 lbs. It was filled with heated air and smoke, furnished by burning wet straw, paper, and the like, under the balloon, the lower part of which was left open to receive it. The balloon rose to a height of more than a mile, but it soon became cooled in the upper regions of the air and fell to the earth.

Give the law of buoyancy. When will a body rise in the atmosphere ? fall? When remain neutral? Examples. (141.) What is a Balloon? account of the early history of ballooning.

When

Give an

In the following August, two brothers, named ROBERT, constructed a balloon of silk saturated with india-rubber, and filled it with hydrogen gas. The ascensional power of this balloon was very great, and being set loose in Paris, it rose with great rapidity, and at the end of four minutes had reached a height of nearly a thousand yards, when it was lost sight of by entering a cloud. It descended fifteen miles from Paris, to the astonishment of the people who saw it.

Manner of filling a Balloon and making an ascent.

142. Balloons may be filled either with hydrogen or with illuminating gas, which is a compound of carbon and hydrogen. On account of the readiness with which the latter gas can be obtained, together with its cheapness, it is generally employed. The envelope is made of silk, rendered air-tight by some kind of varnish, and is strengthened by a network of cords. This network also serves to sustain a wicker basket, or car, in which the aeronaut is seated.

Fig. 105 represents the method of filling a balloon, and preparing it for an ascension. Two masts are erected at a suitable distance from each other, at the tops of which are pulleys. A rope passing through a loop at the top of the balloon, also passes over the pulleys, and serves to raise the balloon during the process of filling.

When the process of filling commences, the balloon is raised till it is three or four feet above the ground, when the gas is introduced by means of a pipe or hose which connects with a gasometer. As the balloon fills with gas it is held down by ropes, and when completely filled, the opening is closed, and the car attached. Care should be taken not to fill the balloon completely, as the gas expands in rising, and unless an allowance is made for this increase of volume, the balloon might be ruptured.

To regulate the ascensional power, the car is ballasted by sand, I contained in small bags. Everything being ready, the ropes are detached, and the balloon ascends with greater or less velocity, according to the ascensional force, that is, the excess of the buoyant effort over the weight of the entire balloon and its cargo.

When the aeronaut finds that he does not ascend fast enough, he increases the ascensional force by emptying one or more of the sand

(142.) With what are balloons filled? Explain the method of filling a balloon. How is the ascensional power regulated?