Jan. 6, iSS3 ] 



SCIENTIFIC NEWS. 



THE MAGIC LANTERN AS A TEACH- 

 ING APPLIANCE. 



THE magic lantern is not as yet sufficiently used by 

 teachers of science, and it is hardly used at all by 

 teachers of geography and history, to whom it might be of 

 great service. Photographs of scenery or ancient m.onu- 

 ments,andanyotheruseful illustrations, whether engraved, 

 painted, or photographed, may be shown in this way to a 

 large class at once. The practical obstacles are reputed 

 to be the costliness of the apparatus, the want of handi- 

 ness in the teacher, and the inconvenience of the 

 darkened room. These difficulties may be lessened by a 

 little consideration of waj's and means, and we shall 

 endeavour to show how to deal with them. First as to 

 -cost. A good lantern is indispensable, but it need have 

 none of the fashionable accessories. Dissolving apparatus 

 and other complications, valued by the exhibitor of 

 phantasmagoria, are entirely useless to the teacher. 

 The price of a sheet-iron lantern, with condenser and 

 photographic lens, is not prohibitive. The manipulation 

 can readily be learned in an hour, and any one who can 

 ■distinguish the taps, and is not too clumsy to slip a flexible 

 tube on to a nozzle, may hope to succeed. No lantern- 

 assistant is necessary. The teacher may easily work his 

 lantern himself, provided that his room is large enough 

 to give a clear working space ot seven or eight feet. 

 The lantern slides are rather expensive (is. gd. to 2S. 

 each) at the usual rates, but amateur photography has 

 improved of late to such a point that it is no longer vain 

 to talk of good home-made slides, and several teachers 

 might be named who turn out every year dozens of 

 excellent lantern-slides. But let the beginner be cautious 

 here. No more execrable object is known to us than a 

 fogged lantern-slide. All defects are magnified, and 

 strike the beholder with horror. " But the darkened 

 room ! " some will say ; "what will become ol the disci- 

 pline of a class when all the lights are turned down to 

 the blue ? How take notes ? How darken the room on 

 .a sunny morning ? " The answer to these questions is, 

 " Don't darken the room at all. Fix the screen in a 

 darkish corner, and if necessary, shade it from super- 

 fluous light with a little black calico, and you will find 

 contrast enough for a small disc illuminated by the oxy- 

 hydrogen light. " It is often a good plan to fix the 

 screen (an opaque screen, of course) in one of the 

 windows ; if used by night, it may be hung in advance 

 of the gas brackets, or beneath a chandelier with an 

 opaque shade just above it. The practical rule is to 

 arrange matters so that the direct source of light, 

 whether gas-jet, or sun, or skj', cannot be seen from any 

 part of the screen. In the next place, If you wish to 

 avoid darkening the room, you must be content with a 

 small disc. A diameter of four or five feet is enough for 

 all but very large rooms, and ample for an audience 

 of loo persons. You will find that the image is softened 

 by the difl'used light, and rendered much more agreeable 

 to the eye. A brilliant light is necessary, and oxygen 

 must be used. Condensed oxygen, thanks tothe barium 

 peroxide method of manufiicture, which is now commer- 

 cially available, is cheap and convenient. Use the iron 

 bottle, and escape all the worry of leak}' gas bags and 

 all risk of the explosion of mixed gases. The oxy- 

 hydrogen lantern, we venture to assure our readers, is a 

 known and proved success when worked on these terms, 

 and will vastly increase the resources of all who need 

 graphic illustration. A little ingenuity, or a question 



now and then put to some practised operator, will remove 

 any difficulty which remains. 



If we have any influence with the makers of magic 

 lanterns, we would urge them to give us for small class- 

 rooms a short-focussed lens, which will give a five-feet 

 disc at about six feet distance. Such a lens would 

 greatly aid any teacher who wishes to work his own 

 lantern. 



WHY DO CLOUDS FLOAT ? 



OF all the paradoxes with which we are famili.3r, 

 there is, perhaps, none which is less generally 

 understood than the flotation and buoyancy of clouds. 

 Whether it is the feathery cirrus or the golden bars 

 and banks of the sunset sky — or 



"A cloud that's dragonish, 

 A vapour, sometime like a bear or lion, 

 A towered citadel, a pendant rock, 

 A foiked mountain, or blue promontory, 

 With trees upon't, that nod unto the world, 

 And mock our eyes with air," 

 we have become so accustomed to disregard their 

 weight, that we seldom wonder why such masses ot 

 vapour should float. 



Their weight is no trifling matter. It is by no means 

 unusual to find half an inch of rainfall in an hour, and 

 this over a square mile means more than three thousand 

 tons, or seven million gallons of water. We may there- 

 fore roughly estimate one of the substantial banks ot 

 cloud, so fancifully described by Antony, to weigh a 

 thousand tons or so. 



There have been not a few conjectures and theories, 

 put forward by well-known men as attempts to explain 

 the puzzle. They began, naturally, in the first place, 

 to examine the structure or the material of which the 

 clouds were composed, and they had little difficulty in 

 most cases in perceiving minute drops ; but here their 

 examination of tacts stopped, and they began to theorise. 

 Saussure and Gay Lussac appear to have abandoned 

 from the first thej idea that the drops were solid. The 

 name "vesicle" was given instead of drop, to begin 

 with, by way of explaining that it was not an ordinary 

 drop, or at all eveftts that it had a " vesicular structure." 

 In plain English, which even now is too often avoided 

 in treating every-day matters from a scientific point 

 of view, the drops were supposed to be hollow, that 

 is, thej' were bubbles. The evidence for this hollow- 

 ness is very small. The idea arose at first in all pro- 

 bability from a desire to find that they were lighter 

 than ordinary drops of water ; the wish was, in fact, 

 father to the thought. But it is fair to note that Saussure 

 himself " saw drops float slowly before him, rather 

 larger than peas, whose coating seemed inconceivably 

 thin." W^ithout for a moment questioning the accuracy 

 of this description, it will be admitted that such 

 "vesicles" are rare phenomena, and are to be observed 

 neither in the steam from a kettle, nor the cloud from 

 a locomotive as it passes under a bridge on which we 

 stand, nor in the chilly evening mist of a low meadow, 

 nor do we find them on the rarer occasions on which 

 we are at close quarters with an ordinary cloud on a 

 mountain. 



But were it proved that clouds are composed of minute 

 bubbles we should be no nearer the solution of the 

 problem, for not only would a pound of bubbles weigh 

 as much as a pound of water, but owing to the elastic 

 tension of the film the air inside each bubble would 



