I'NKI'MATK'S. 





PNEl' MATH'S. III. 



BUCKKR SPXLE-PM 8TPUOM CUP OF TANTALUS INTER- 

 MITT K.Vf Hl'HINUB 8ABBATIO KIVKK BAUOMKTKit. 



THE pressure of the air furnishes us with explanations of many 

 of the common phenomena of overy-day life. When a oaak is 

 tap IKK! , the beer noon ceases to flow unless a small hole be bored 

 at the top, just aa in the magic decanter the wine refined to run 

 out of the Hinull hole at the bottom until the air wan admitted 

 by the loosening of the stopper. A well-made cask U perfectly 

 lur-t i^lit, and an aoon aa a little of the beer has been drawn from 

 ;r m-ido becomes rarefied; the pressure of the external 

 air in therefore greater, and, acting on the li<i'ii<l in the' tap, 

 prevents its flow. Aa soon, however, as air is admitted by the 

 Hpili'-pog, the pressure is equalised, and the beer flows evenly. 

 For the same reason a small hole is usually made in the li i i .<. 

 HO that when the water standing round the lid makes it 

 nearly-air-tight the tea may still flow in an even stream. 



If we invert a wine- bottle, the liquid will flow out in a very 

 irregular way ; the air has to pass in by the neck at the same 

 time as the contents flow out, and the meeting of the two pro- 

 daces the well-known gurgling sound. If, however, the bottle 

 bo slightly inclined, so that the air may enter at the upper 

 part of the neck while the liquid flows out at the lower part, all 

 this gurgling is avoided, and the liquid flows better and more 

 npidly. 



Wo have another illustration of the pressure of the air in the 

 common sucker, so well known to every boy. A piece of string, 

 with a knot at the end of it, is passed through the centre of a 

 circular piece of thick leather, which has teen soaked in water 

 till it has become quite soft and pliable. It is then pressed 

 closely down on a stone or flat surface so as to exclude the air 

 from under it, and will be found to adhere so firmly that the 

 stone may be raised by the string without its leaving the leather. 

 The reason of this is that the moisture prevents the air entering 

 between the stone and the leather. When, therefore, the leather 

 is raised in the middle by the string a partial vacuum is pro- 

 duced, and the pressure which the air exerted on the upper 

 surface of the stone is transferred to the leather and balanced 

 by the tension of the string. The pressure of the air on the 

 under side of the stone, being no longer balanced by 

 a corresponding pressure above, lifts the stone. 



It is frequently required to draw a small quantity of 

 wine from a cask, as a sample, without tapping it : a 

 small instrument known as a wine-taster is therefore 

 made nee of. This consists of a hollow tubular vessel, 

 having a small aperture at each end, and somewhat 

 bulged in the middle, as shown in Fig. 5. The bung 

 is removed from the cask and the taster inserted. 

 The wine soon rises through the opening at the lower 

 end till it finds its level, the thumb is then placed 

 over the opening above, and the air being thus pre- 

 vented from entering, the wine is retained, and can be 

 removed from the cask without any loss. On the 

 thumb being removed from above, air will enter, and 

 the liquid will Sow out into a glass. 



Bird-cage fountains also depend for their action on 

 the pressure of the air. The reservoir is constructed 

 so as to be air-tight, and a small trough is placed near 

 tho lower end from which the birds may drink. This 

 communicates with the reservoir by a small hole made 

 at the level at which it is desired to maintain the 

 water. As soon as the water is so far removed 

 from the trough that this hole is exposed, a bubble of air enters 

 and displaces a small quantity of water, and in this way a uni- 

 form level is maintained, and a constant supply of fresh water. 

 Pneumatic inkstands have been constructed on a similar prin- 

 ciple, the advantage resulting from their use being that a much 

 less surface is exposed to the air, and therefore the ink does not 

 thicken so rapidly, and also that there is less danger of spilling 

 the contents. 



We must now pass on to notice a very useful piece of ap- 

 paratus the syphon. 



In many manufacturing processes, and in chemical experi- 

 ments, a liquid is often allowed to settle, and the clear liquor 

 above has to be drawn off without disturbing the sediment, as 

 would be done if the vessel were tilted so as to pour out the 

 liquid. This can easily be accomplished by means of the 



Fig. 5. 



syphon, which eonsfcts merely of a tnbe bent into the bap* of 

 the letter U, one limb, however, being longer than the other. In 

 Fig. 6, o is the vessel from which the wter has to be drawn off. 

 and c B represents the syphon. This u filled with water, Mid 

 then, the ends being dosed by the finger, inverted into the 

 vessel ; or it may be placed 

 in the vessel, and the air 

 exhausted from it by sock- 

 ing with the month at the 

 end B ; the air being thus 

 partly removed from the 

 tube, the water rises and 

 flows over tho bend. The 

 principle on which it acts 

 is simply this : Let the sy- 

 phon be full of water, and 

 let us imagine a layer of 

 water across the highest 

 point of the bend to be- 

 come solid. The forces 

 which act on thin layer and 

 force it towards are the 

 pressure of air acting up- Tl?. 6. 



wards at B and the weight 



of a column of water equal in height to D c, for it is clear that 

 this column has to be supported by the layer. The forces which 

 drive it in the other direction, or towards B, are the pressure of 

 the air on the surface of the liquid in c, which pressure is trans- 

 mitted along the tube, and is just equal to the pressure at B, and 

 the weight of a column of water equal in height to A B. It will 

 thus be seen that the liquid will be moved along the syphon to- 

 wards B with a force equal to the pressure of a column of water 

 whose height is equal to the difference between the two column* 

 A B and c D. Hence, the greater the difference of level between 

 the liquid in the vessel c and the open month B of the tube, the 

 greater will be the speed at which the liquid will flow ; if both 

 come to the same level it will cease to run. In order to avoid 

 the risk of drawing some of the liquid into the mouth while 

 sucking out the air, a small tnbe is frequently attached near the 

 end of the lower limb of the syphon ; the finger may then be 

 placed over the end to close it, and the air drawn out by this 

 tube, in which the rise of the liquid will be plainly seen. 



An arrangement, designed to avoid trouble in filling the tnbe, 



is known as the Wurtemberg syphon. In this both limbs are of 



tho same length, and are turned up so that, when once filled, it 



| will remain so, and thus is always ready for use. As, however, 



j the only difference in the effective length of the limbs arises from 



the distance to which one is immersed in the liquid, only a small 



head of water can be obtained, and thus the liquid flows but 



slowly. 



Since it is the pressure of the atmosphere which causes the 

 liquid to rise in the bend of the syphon, the highest point in it 

 must not exceed 34 feet if water is used, or 30 inches if it be 

 mercury ; for, as we have seen, the pressure of the air is only 

 sufficient to balance columns of these heights. 



In carrying on extensive drainage operations, as is frequently 



done in the low parts of Holland, or, on a smaller scale, in the 



fen district of our own country, a drain is frequently constructed 



which has to discharge itself into a tidal river, or the sea, but 



is, at high water, below its level. Means have, therefore, to be 



adopted to prevent the tidal water flowing up the drain and 



flooding the land, and floodgates are frequently used for this 



1 purpose. They are, however, very expensive to make and keep 



in repair on account of the great pressure they have to sustain 



I from the water being higher on one side than on the other ; and 



! if a channel, however small, be formed by the water under their 



! foundations, its pressure will soon be sufficient to blow up the 



sluice. Thi< happened a few years since at the " Middle Level," 



one of the drains which empties into the Onse nesj King's Lynn. 



and a great loss of property was the result. To guard against 



these difficulties, syphons are now sometimes used instead of 



gates, and are found to answer well. Piles are driven across the 



drain, and a strong embankment formed, so as to resist the 



pressure on either side. Large syphons are then placed over 



this, and small pipes are brought from the highest point of each 



syphon to a large air-pump, by which the air is removed, and the 



syphons set to work. When the water rises higher on the river 



I side, air is re-admitted above, and the syphons oease to act. 



