ADDENDA.] 



MECHANICAL PHILOSOPHY. PNEUMATICS. 



777 



R and the corresponding end-face of the cylinder, or it 

 is very uearly equal to one-half the length of the 

 cy Under. 



In an upward stroke, the air above the piston A is 

 propelled through the valve V into the atmosphere, 

 while a vacuum is being formed beneath the piston B. 

 When the piston A strikes against the top of the 

 cylinder, the air from the receiver rushes through the 

 pipe R, and diffuses itself through the lower half of the 



cylinder. In a downward stroke, the air beneath the 

 piston B is propelled through the valve v into the at- 

 mosphere, while a vacuum is being formed above the 

 piston, A, and so on. It will be observed that the double 

 piston performs a double duty at every single stroke ; 

 for while a vacuum is being formed in one half of the 

 cylinder by one piston, the other piston is propelling the 

 air from the opposite half, into the atmosphere ; hence 

 much time and labour are saved. 



ADDENDA. 



THE PATENT ICE MAKING MACHINE. In 

 temperate climes, sufficient ice is produce.d for summer 

 consumption during the winter season ; but in tropical 

 countries, where it is unknown as a natural production, 

 and its value is often at a high price, a method of pro- 

 ducing it at pleasure has long been a desideratum. 



In our Section on Heat we have given a list of 

 freezing mixtures : these, however, can only be employed 

 on the small scale.* It has, therefore, been frequently 

 attempted to apply the air-pump to refrigerating 

 purposes ; and, perhaps, the most successful machine of 

 the kind, is one which was shown in the Exhibition 

 of 1 %2, and of which we shall give a short description. 



It has been already stated, that ether boils at a very 

 low temperature in a vacuum, and that if a little be 

 plaf> -I in a watch-glass, resting in another containing 

 water, the latter may easily be frozen in the receiver of 

 an air-pump, whilst the air is being exhausted in the 

 usual manner, t On the large scale, however, other 

 arrangements lor applying this principle must be adopted, 

 both on account of speed and economy ; for the atten- 

 uated atmosphere, being but a poor conductor of heat, 

 would freeze but slowly any quantity of water exposed 

 to its action ; and the ether, which is an expensive 

 material, must be recovered if possible, so as to be used 

 ; repeatedly for the same purpose. The latter difficulty is 

 overcome by permitting the restoration of the atmospheric 

 pressure, when the ether regains its liquid form, which it 

 had lost in the vacuum. To effect the rapid congelation 

 of the water, strong brine is employed, being first cooled 

 by the evaporation of ether in a receiver, from which the 

 air is pumped by a steam-engine. The brine is a good 

 conductor of heat ; and, when thus cooled, it is caused 

 to run round square cells containing the water to be 

 frozen. The cells are placed at a little distance from 

 each other, in a long trough, as represented in the folio 

 engraving ; and one of them is seen suspended over the 

 trough on the right hand of the plate. The circulation 

 of the brine ia constantly kept up ; fresh cold liquid 

 being driven in by a pump, as that which has cooled the 

 water is withdrawn. In a short space of time the water 

 in the square cells is frozen, through the abstraction of 

 its latent heat by the brine, and it may then be removed 

 in solid flat cakes of ice. Fresh water is then introduced 

 into the emptied cells ; and thus the freezing process 

 can be kept up without intermission, so long as the 

 steam-engine is kept at work. 



Comparatively speaking, in this climate the above is 

 an expensive process ; but when employed in hot 

 countries, it produces ice in large quantities, at a price 

 which is much lower than that hitherto charged. The 

 machine has the advantage of requiring no chemical salts, 

 as in the old method : in fact, the whole process is one 

 of a purely mechanical nature. 



BALLOONS. In connection with the subject of 

 Pneumatics, a few remarks on balloons may prove 

 interesting to many of our readers. The ascent of a 

 balloon in the air, is owing to the fact, that the contents, 



See ante, p. 37. 



t See ante, p. 21 



bag, car, &e., weigh less than the bulk of air they 

 occupy. In other words, the balloon and its appendages, 

 taken together, have less specific gravity than the atmo- 

 sphere in which they float. This would, at first sight, 

 appear paradoxical, but it is easily explained. Balloons 

 may be raised by either of two means. If a large bag of 

 paper, closed in all parts, except at a small oritice in its 

 lower end, have the air it contains rarefied, by holding it 

 over a gas flame, so that the air inside shall become 

 heated ; then, a less weight of hot air having the same 

 bulk as cool air, will fill it. If the difference of weight 

 between the heated air and that of the external atmo- 

 sphere, bulk for bulk, equal that of the bag containing 

 it, the bag will float in the air. On this principle the 

 common fire-balloon is made to ascend. A piece of 

 cotton wool, wetted with spirits of wine, is hung at the 

 orifice of a large bag of tissue paper ; and the spirits 

 being inflamed, the internal air becomes rarefied, and the 

 balloon ascends. The earlier attempts at aerostation, 

 made by Montgolfier and others, were carried on in a 

 similar manner. We need scarcely state that such a 

 plan was highly dangerous. Large quantities of straw 

 were burnt beneath the opening of a silk bag of 

 considerable size, and thus the air it contained became 

 rarefied. Occasionally, however, the machine caught 

 fire, and fatal results occurred to those who had made 

 the ascent. 



The use of hydrogen was subsequently suggested ; and 

 it is a familiar lecture-table experiment to fill small 

 balloons with that gas, when they immediately ascend in 

 the air.J Now, as pure hydrogen has a specific gravity 

 of only, in round numbers, - 07, whilst air (the standard 

 of gases) = 1-00, it follows, that any bulk of hydrogen 

 will only weigh one-fourteenth part of the same of 

 atmospheric air. Hence, if a light and air-tight material 

 be shaped into any hollow form, so that when filled with 

 hydrogen, its weight, and that of the contained gas, shall 

 be less than that of an equal bulk of air, of course it will 

 at once fly upwards on being set free. 



On the large scale, coal gas is always employed in 

 filling balloons, because it is much cheaper, and almost 

 always readily obtained. The external skin forming the 

 balloon, consists of fine silk, which has been well var- 

 nished, to render it air-tight. A car is attached beneath 

 it, by means of netting and cords. And such arrange- 

 ments have been made of sufficient size to permit of the 

 ascent of several persons. 



Many attempts have been made to steer balloons, but 

 all such have proved failures. At the present time, 

 their use is confined to occasional ascents for scientific 

 and military purposes, and as an attraction at places of 

 public amusement. Balloons were frequently employed 

 during the civil war in America, in 1862-'63 : the British 

 government have instituted experiments to test their 

 value for reconnoiting purposes. Some account of the 

 ascents of Mr. Glaisher, for meteorological observations, 

 will be found in Chapter V., in the Section on Meteor- 

 ology, p. 1174, t. seq. EDITOB. 



J See ante, p. 320. 



VOL. I. 



