♦ KNOVS^LEDGE . 



[Aug. 17, 16 



adjustment by a slow screw movement ; and having set 

 A B across the cup, and levelled the cup's surface in direc- 

 tion A B (nothing very exact is needed in this direction), 

 slowly shift the surface in direction square to A B by 

 means of the screw movement, until looking down the face 

 A C its image on the mercury is reduced to a straight line. 

 Then the face A B is perfectly vertical. Now, as before 

 look into the steel mirror A C, bringing the centres of the 

 pupils to the edge A B, as at a, b. The sea horizon will 

 then be seen as at cd about a quarter of an inch below 

 A B, if the eyes be about two yards from the mirror. 



For different heights, different depressions of the sea 

 horizon will be noted in this experiment. For places near 

 the sea level there is no observal)le displacement of e/ 

 below a h ; for places much higher than the 200 ft. of our 

 experiment the displacement is much greater. 



Let us see what is the law connecting h the height of 

 the observer, with the angular depression c of the sea 

 horizon. Call the radius of the earth r. The angle c 

 is the angle ft a B of Fig. 8, or (appreciably it is the angle 

 subtended by B b, or twice h, at a distance a B. Now 

 (aB)- = 2rA (appreciabl^y) 



„ B6 ■2h /2A , ... , '^ 1 r 



/. c= -B= — A / , where a right angle = ;^= 1 -.5 



Thus the angle of depression varies as the square root of 

 observer's height. In the above, c is the geometrical depres- 

 sion-angle. The apparent depression (which may, indeed, 

 be called the real depression, since it is what is observed) 

 that we are inquiring about is about three-fourths the 

 geometrical depression. 



Let us inquire what height is required to give an 

 observed depression of one degree, which would be very 

 slight — far too slight to be noticed without the aid of 

 Instruments. The above equation gives us, since c, the 

 geometrical depression in this case, is four-thirds of a 

 degree. 



O- V Qfl 



5708 



■f- circ. measure of 1 de; 

 unit of length. 



39G0 



1 



taking a mile as our 



Now, circular measure of l° = -r^(l'.5708) = '01745 

 yo ' 



. 16 , 2 h 



•■• squaring, — {Ol'io)—^^^ 



say (roughly) 16 (-000.3045)=:^ 



or A= 3520 X -000.3055 

 = 1'0754 of a mile 

 = 5678 feet 



One may say that by ascending to the height of a mile 

 a depression of one degree (apparent) may be observed, if 

 the sea horizon is visible. To attain a depression of two 

 degrees a height of four-and-a-half miles must be attained. 

 This angle would seem very small, scarcely to be recognised 

 by the unaided eye, even if the sea horizon were visible. An 

 ordinary landscape horizon would not seem lowered at all 

 recognisably. Hence the singular optical illusion by which 

 the region visible below a balloon at a great height seems 

 shaped like a vast basin. 



(To he confitiued,) 



One Shulin-g each will be paid for copies of the Index to 

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THE AMATEUR ELECTRICIAN. 



BATTERIES.— III. 



YET one more " typical " cell before we enlarge on some 

 of the many hybrid cells more or less useful to the 

 amateur in his various experiments. 



The Leclanche cell, the form of battery upon which it is 

 our present purpose to dwell, is one which is constantly 

 increasing in its sphere of usefulness, and which will doubt- 

 less long continue to hold its own against many of the so- 

 called improvements which the recently increased demand 

 for electrical sources have produced. "The cell consists of 

 an outer vessel, generally (for convenience only) of glass ; 

 inside this is a porous pot, which contains the negative 

 element. The positive element is in the older patterns a 

 circular rod of good but unamalgamated zinc, immersed in 

 a saturated solution of sal ammoniac or ammonic chloride 

 (NH^Cl or AmCl). The negative element consists of 

 a rod, plate, or block of gas carbon, embedded in a mix- 

 ture of the same material, crushed to about the size 

 of peas, and similar-sized manganic peroxide or black oxide 

 of manganese (MnOo). The last-mentioned is one of 

 those substances so bewildering to the tyro, on account 

 of the multiplicity of their names. The mixture, 

 which should nearly fill the porous pot, should contain 

 approximately equal quantities of its constituents and 

 should be free from dust or very small pieces. A piece 

 of paper, or some such substance is then fitted in the 

 pot over the mixture, and pitch marine-glue, or other 

 impervious material poured over it, so as to keep everything 

 in its place, one or two holes, being, however, provided to 

 facilitate the escape of polarising gases. It will be seen 

 that no liquid is placed in the porous pot, the solution for 

 which is provided by a portion of the sal ammoniac solution 

 passing through the pores of the pot. As the function of 

 the carbon rod is simply that of a conductor, it is apparent 

 that its dimensions are immaterial, the chief feature in 

 connection with it being the necessity for providing it with 

 a cap and connections unassailable by ammonia fumes and 

 solution. 



When the zinc and carbon connections are joined, 

 chemical action commences, the zinc is dissolved, and 

 converted into chloride of zinc (ZnCL) by combination 

 with the chlorine of the sal ammoniac. The other portion 

 (N Hj ammonium) of the sal ammoniac is further decom- 

 posed into ammonia (N H,) and hydrogen. The hydrogen, 

 which is in what is known as the nascent condition, attacks 

 the manganic peroxide, reduces it to a lower oxide (sesqiL- 

 oxide of manganese, Mn.,0..), and combining with tie 

 released oxygen, forms water (OH.,). The series of «x- 

 changes may be represented by equation thus : 



Zn-f 2NHjCl+2MnO,=ZnCl„-|-2NH3 + OH.,-f Mn/'s. 



Those of our readers who are versed in chenical 

 notation will see that for every volumetric equivaleit of 

 zinc dissolved, two equivalents of both the sal amnnniac 

 and the manganic peroxide are decomposed ; the sohtion 

 gradually losing its character and becoming instad a 

 solution of chloride of zinc and ammonia, while thf man- 

 ganic peroxide is gradually reduced to a brownish pwder 

 and tends in time to make the mixture in the porois cell 

 mudd}' and more or less impervious, preventing ali;e the 

 advent of fresh solution and the exodus of unconbined 

 hydrogen. 



It will be seen, on examination, that the Leclan;h6 cell 

 differs very widely from either the Daniell or theBunsen 

 cell. In the Daniell, it may be remembered, we lave tie 

 hydrogen from the zinc division attacking suljhate of 

 copper, forming sulphuric acid and copper, thf latter being 

 deposited on the negative, or copper, plate, whi;his there- 



