34 



♦ KNOWLEDGE . 



[July 11, 1884. 



a closed pane contiguous to the open one. A most beau- 

 tiful experiment, illustrating both refraction and total re- 

 flection (for the idea of which we are indebted to a little 

 book on " Light," by Mayer and Barnard, published by 

 Macmillan ct Co.), will probably come in here most appro- 

 priately. Fig. 17 is intended to illustrate it. 



Fig. 17. 



The reader should buy the biggest globular glass flask 

 <;hat he can obtain, and have a hole drilled in its side at 

 the shop where he buys it. Stopping this hole with a 

 cork, the bottle must be filled with water, and a parallel 

 'beam of light from a kerosine lamp and a bull's-eye lens be 

 projected on to the side of the flask, in the way illustrated 

 by our figure. It must be so managed that the refraction 

 shall bring the rays of light, of which the beam is composed, 

 to a focus on the cork, and the lamp should be enclosed in 

 a box, or otherwise so surrounded by opaque material, that 

 the room would be dark but for the light passing into the 

 bottle. This condition of things being secured, a pail may 

 be placed on the floor, and the cork taken out of the side 

 -of the bottle ; when a most beautiful etfect will be per- 

 ceived. The light will be totally reflected from the inner 

 surface of the issuing jet of water, which will appear like 

 a stream of molten silver, and the interposition of coloured 

 glasses between the source of light and the bottle will 

 tinge this of their own hues. Of course, sunlight or the 

 •electrical light will give even more brilliant results still. 

 It was by the aid of the latter, projected in this way 

 through variously-coloured media, that the so-called " Fairy 

 Fountain " was managed which was exhibited at the 

 ■defunct Panopticon in London. 



In our next paper we shall enter upon the subject of 

 refraction through successive surfaces, which, unlike those 

 ■of the window-pane spoken of above, are inclined to each 

 other. 



The couvers.azioue of the Societj- of Tele<rraph Engineera and 

 Electricians was held at King's College last Thursday evening. 

 Several highly interesting pieces of apparatus were exhibited, 

 including an artificial cable, rejiresenting, in resistance and 

 inductive capacity, about 8,000 miles of submarine line, viz., about 

 16,000 ohms and 2,450 microfarads, by Latimer Clark, Mnirhead, & 

 Co. (Limited) ; Professor Dr. Dvorak's acoustic experiments, 

 showing the conversion of sonorous vibrations into a continuous 

 current of air ; and the model Paris legal ohm (sent by Professor 

 Mascart), by W. H. Preece, F.E.S. In the Physical Lecture Theatre, 

 Sir. H. Tomlinson exhibited and explained experiments on the 

 effects of mechanical stress on the electrical resistance of nickel, 

 which showed that up to a certain point the stress reduces the elec- 

 trical resistance of nickel, notwithstanding the fact that the wire 

 IS increased in length and proportionate!}- decreased in sectional 

 Sfea. 



THE INTERNATIONAL HEALTH 

 EXHIBITION. 



VII.— WATER AXD WATER-SUPPLIES. 

 (Continued from p. 9.) 



RECEXT researches in the natural sciences have one 

 characteristic of value in common, they commence 

 with the simplest condition of the individual, and trace it 

 step by step through the progressive stages of its develop- 

 ment and degeneration. A somewhat analogous process 

 to that of the evolutionist might be applied here in the 

 record of the cycle of events which constitute what we 

 may term the life-history of a drop of water. 



The principle of the conservation of matter teaches us 

 that do what we will we cannot create, nor can we destroy, 

 a single atom. The water at present in our terrestrial 

 globe eipials in quantity the water of the mystic age when 

 all was void or nebular ; but the changes to which it has 

 been subjected through countless periods remind us that 

 although matter is immutable, that other great entity 

 called energy can, through its transformations, remodel 

 it, as it were, until its original forms become obscured. 



In our universe we always start with something tangible ; 

 in this case let us commence with a drop of water, and try 

 to interpret some of the varied phenomena which are to be 

 attributed to its agency. In this way shall we learn what 

 a powerful sway it exercises over the destinies of our 

 earth, how it helps to endow it with that which distin- 

 guishes it as a livinij in contradistinction to a </<-«</ world, 

 and how we are enabled to find therein a striking exem- 

 |)litication of the laws of matter and of energy which we 

 have just cited. 



The heat of the sun acting upon the waters of the 

 earth, whether they aie in the form of oceans, rivers, 

 snow-fields, or glaciers, causes the original solid or 

 liquid to give place to the gaseous condition. The invi- 

 sible vapour thus produced rises in the atmosphere 

 according to the law of ditfusion of gases, and remains 

 thus uutil the air containing it cools below its point 

 of saturation, or dew-point. A change then takes place, 

 which is usually brought about in two principal 

 ways ; either by contact of a colder current of air 

 with the moisture-laden stream, or by the rise of the 

 latter into the colder upper stratum of the atmosphere. 

 The condensation takes place through deposition 

 on free surfaces, either by contact with the cold 

 earth or mountain sides, or if at a great elevation., 

 with excessively minute particles which pervade the 

 air,* thereby giving rise to the formation of mists and 

 clouds. In any case vapour is formed, and remains sus- 

 pended in the atmosphere in tiny particles of liquid, or if 

 the temperature be low enough, as aggregates of icy dust. 

 A continuation of the process adds to the sizf' of the 

 cloud particles, until they can no longer be suspended, and 

 hence descend upon the earth, in virtue of gravitadon, as 

 rain, sleet, hail, or snow. The theory of condensation 

 around nuclei of cosmic or other origin, divests rain, even 

 in the uppermost regions, of its character of absolute purity, 

 and by the time it reaches the earth, it no longer bears the 

 slightest semblance to chemically pure water, but is highly 

 charged with matters, both solid and gaseous, which are 

 destined to create profound changes durmg its action as a 

 geological agent. We will not here consider the progres- 

 sive stages undergone by water which comes to the earth 

 in a solid form to feed the snow-clad regions of its lofty 

 mountain ranges, as, after all, it resolves itself ultimately 



* Coulier and Mascart, " Xaturforsoher," 1875, p. 400 ; Aitken, 

 Proc. Roy. Soc. Edin., December, 1880. 



