318 



THE POPULAR EDUCATOR. 



Fig. 7. 



An ingenious scientific toy, known as the Cup of Tantalus, I 

 has been constructed, and acts on the same plan as the common ' 

 syphon. It is, in fact, an intermittent syphon, and is useful as 

 serving to explain the action cf intermittent springs. A eyphon 

 is inserted in a cup so that its longer limb may pass through an 

 aperture at the bottom, and the highest point of the bend may 

 be rather lower than the brim. If water be now allowed to run 

 into the vessel, it will fill as usual until the water reaches the 

 level of the bend. The syphon will then begin 

 to act, and if its size be the same as that of the 

 supply-pipe, the water will remain at that level ; 

 if, however, the syphon carry it off more rapidly, 

 the vessel will be emptied ; the syphon will then 

 cease to flow, and the vessel will again be filled. 

 An intermittent flow will thus be produced. 

 Various modifications may be made in the con- 

 struction of this vessel. Sometimes the handle 

 is made hollow, and thus serves as a syphon, and, 

 when thus made, the reason of the cup emptying 

 is not so easily seen. Sometimes, too, an open 

 tube is inserted in the vessel, and another, closed 

 at the upper end, is inverted over it : all, how- 

 ever, act on the same principle. 



We shall now be able to understand better the action of 

 intermittent springs, many of which exist in different parts of 

 the world. In England there is one known as Weeding Well, in 

 the Peak of Derbyshire ; others exist at Giggleswick, in York- 

 shire, and near Torbay ; but the most noted of all are found in 

 Palestine. 



Josephus speaks of a stream called the Sabbatic river, which 

 flows one day, and then is dry for the next six days. Pliny 

 refers to the same ; but he makes it flow for six days, and rest 

 on the seventh. The existence of such a river was long 

 doubted ; but modern travellers say that they have discovered a 

 small stream which seems to be that referred to. Now, however, 

 it is dry for two days, and flows on a portion of the third ; 

 but this alteration may be easily accounted for. The annexed 

 diagram will serve to explain the action of the spring. A largo 

 reservoir is supposed to exist in the 

 hill from which the stream issues. 

 This is supplied by the rain, which 

 percolates through the sides of the 

 mountain and, by various inlets, finds 

 its way into the cavity. 



A syphon-shaped channel is also 

 supposed to exist, of such a capacity 

 that it can carry off the water more 

 rapidly than it enters by the different 

 feeders. Now it is clear that the 

 water will go on accumulating, but 

 none will flow till the cavity is filled 

 to the level of the bend in the chan- 

 nel. As soon, however, as it attains 

 this level, the syphon will begin to 

 act, and the stream will flow until the 

 reservoir is empty, when air will enter 



the syphon, and it will cease to act until the cistern shall again 

 be filled, when the same effects will be repeated. 



The smaller the cistern, the more frequently will the water 

 flow. Hence it is quite possible that the statement of Josephus 

 about the Sabbatic river may have been true, but that the 

 cistern has been gradually filling up, so that now it flows once 



in three days instead of once in seven. An enlargement of the 



channel by which the water issues, or an increase in the supply ! tion to its mode of construction and action. 



fourths ol the height of the vessel, and then passes through the 

 side. The water is allowed to enter in a constant stream by a 

 pipe about half the size of the syphon, and so placed that the 

 water enters in such a direction as to keep the contents in a 

 constant state of motion. As soon as the vessel is filled to the 

 level of the bend, the syphon commences to work, and in a 

 short time empties the vessel, the false bottom causing the 

 photographs to be left quite dry, a thing of great importance, 

 as, thereby, the last traces of the chemicals em- 

 ployed are more easily removed. The vessel 

 then fills again, to be once more emptied in the 

 same way. 



We have now noticed several important results 

 of the pressure of the air, and the construction 

 of machines which act by means of it ; but wo 

 have not yet seen the mode of determining how 

 great this pressure really is. We have, however, 

 stated it to amount to about 14ilbs. per square 

 inch, and must now show a proof of the fact. 



We might take a surface of known area, and 

 having, by means of an air-pump, removed the 

 air from under it, ascertain the pressure by a 

 spring balance. This plan would, however, bo 

 very difficult and uncertain, as it is impossible perfectly to re- 

 move the air, and it would be very difficult to ascertain the 

 pressure exerted on the balance. There is, however, another 

 mode of ascertaining this, which depends on the fact that a liquid 

 transmits pressure equally in all directions. If we take a glass 

 tube about a yard long, sealed at one end, and, having filled it 

 with mercury, place the thumb over the open end, and invert it 

 into a cup of mercury, we shall find that a small part of the 

 fluid will run out, but that the tube will remain filled to a height 

 of about 30 inches above the level of the mercury in the cup. 

 This experiment was first performed about the middle of the 

 seventeenth century by Torricelli, after whom the empty space 

 left at the top of the tube is known as the Torricellian vacuum. 

 Now, if we consider the forces at work, we shall see that tho 

 air presses on the surface of the mercury in the cup, and its 

 pressure is transmitted through this 

 to the mercury in the tube. Tho 

 tipper part of the column ia, however, 

 shielded from this pressure by the 

 closed tube, and, since the whole is in 

 equilibrium, the pressure produced by 

 the air must be exactly equal to that 

 produced by the weight of a column 

 of mercury 30 inches high. Let us 

 suppose the tube to have an area of 

 one square inch : the pressure then 011 

 this area, and accordingly on every 

 equal area, will be equal to the weight 

 of 30 cubic inches of mercury. Now 

 a cubic inch of water weighs 252'5 

 grains, and the specifie gravity of 

 mercury is 13'59 ; a cubic inch of it 

 weighs, therefore, about 3441 grains, 



and 30 cubic inches weigh about 14|lbs. This, therefore, is tho 

 pressure exerted by the air on every square inch of surface when 

 the mercury stands at a height of 30 inches. In this climate 

 the mean height is rather under this, being about 29'9, and the 

 pressure, therefore, is a little over 14|lbs. 



This simple instrument is one of the most important in tho 

 science of Pneumatics ; we must, therefore, give a little atten- 



It is called the 



brought by the feeders, would produce the same change. 



The Pool of Siloam is another instance of a spring of this 

 kind. Dr. Robinson states that, when he was there, he observed 

 the water rise nearly a foot in five minutes, and that he was 

 informed that such rises occurred frequently, sometimes two or 

 three times in the course of a single day, but at other periods 

 only two or three times a week. 



An ingenious application has been made of this principle in 

 an apparatus constructed for the purpose of washing photo- 

 graphs. In order to ensure permanency in prints it is requisite 

 that they should be well washed, and the more frequently the 

 water is changed the better will this be done. A vessel is 

 therefore made with a small depression at the bottom separated 

 from it by a grating. From this a syphon rises about three- 



Barometer or " Weight-measurer," though, in reality, it is the 

 pressure and not the weight of the air which it records. 



That it is the pressure of the air which supports the column 

 of mercury is easily seen, by the fact that if we make an open- 

 ing so as to allow the air to press on the surface of the mercury 

 in the tube, it will immediately fall to the level of that outside. 

 A more conclusive experiment as to this point was devised by 

 Pascal. He said that if it was the weight of the air which sup- 

 ported the column, then, if the barometer were taken to any 

 elevation so as to leave a part of the atmosphere below it, the 

 mercury ought to stand at a less height. The experiment 

 was accordingly made. The instrument was conveyed up a moun- 

 tain, and the height noted at intervals, when it was found, aa 

 predicted, to diminish gradually, as the elevation increased. 



