56 



♦ KNOWLEDGE ♦ 



[July 18, 1884, 



Mediterranean were not icebound, it would be sufficiently 

 difficult to conceive that the glaciers formed around its 

 shores would pass out in stately procession through the 

 imagined straits south and west of Cape Horn. How 

 should currents sufficiently strong be generated to bear 

 these glacial masses away ? How could collisions, blocking 

 up the mouth of the strait, often for months together, be 

 avoided ? And when the consideration is added that an 

 Antarctic Mediterranean would almost certainly be frozen 

 over the whole year through, the theory that it is within 

 such a sea that Antarctic glaciers are formed becomes, in 

 our opinion, altogether untenable. If such a sea exists, it 

 must be blocked up with ice too completely for any con- 

 siderable movements to take place within it. Even the 

 glaciers on its borders must be unlike the glaciers known 

 to us, because the downward motion of the ice-masses 

 composing them must be so checked by the resistance of 

 masses already accumulated, as to be scarcely perceptible 

 even in long periods of time. 



(To he continued.) 



THE ABSOLUTE CAPACITY OF A 

 CONDENSER. 



AT the last meeting of the Physical Society, Mr. R. T. 

 Glazebrook, M.A., F.R.S., of Cambridge University, 

 described some interesting experiments he had made to 

 determine, in absolute measure, the electrical capacity of a 

 condenser sent to him for the purpose by Messrs. Latimer 

 Clark, Muirhead, & Co., the well-known makers. The 

 general method employed by Mr. Glazebrook is that given 

 by Clerk Maxwell in his treatise on "Electricity," vol. II., 

 sec. 776. A commutator driven by a tuning-fork is fitted 

 to the condenser in such a manner that the plates of the 

 ■condenser are alternately connected to two points at dif- 

 ferent electrical potentials, and then put into communication 

 with each other. The condenser and commutator are inserted 

 as one of the arms of a Wheatstone bridge, and Maxwell 

 has shown that if the period of vibration be sufficiently 

 slow, the combination is approximately equivalent 



to a resistance of . — where n is the frequency of the 



tuning-fork, and C the capacity of the condenser. Thus, 

 if a be the resistance of the arm conjugate to the con- 

 denser, c and d resistances of two other conjugate arms, 



we have the condition for a balance — = c d, or w C =: 



?iC 



— -. Thus C can be found if a, c, d, and n be known. Mr. 

 c a, 



J. J. Thomson has, however, shown ("Phil. Trans.," part 

 iii., 1883) that Maxwell's formula is only approximate, 

 and has given the correct formula. It was this which Mr. 

 Crlazebrook used in his tests, and the arrangement of 

 apparatus is shown in the figure. The condenser 

 and commutator were placed on the bridge arm B D, 

 and P is the moving piece commutator, which, when 

 in contact with S, changes the condenser, and, 

 when in contact with R, discharges it. The tuning- 

 forks used had frequencies of 16, .32, 64, and 128 to the 

 second, as determined by careful comparison with a clock 

 by the method of Lord Rayleigh. The corresponding 

 values of the capacity were, in terms of the legal ohm., 

 •3336 mf. (microfarads), -3340 mf., -3335 mf., and -3337 

 mf., the mean being 3337 microfarads. The experiments 

 do not show any variation in the capacity, as the time of 

 charging is changed from 1-1 6th to 1-1 28th of a second. 



The formula also gives a ready and accurate means of 

 determing the pitch of a tuning-fork, for if the capacity of 

 the condenser used is known, the value of n can be deter- 

 mined. Mr. Glazebrook has successfully used it for this 

 purpose. A question arose at the meeting as to the effi- 

 cacy of mercury contacts in such experiments, and Dr. 

 W. li. Stone stated that he found mercury and iron con- 

 tacts to be free from sticking. Whether this is an advan- 

 tage or not in making a good contact is doubtful. Recent 

 experiments within our knowledge would seem to point to 

 a microphone action in such contact?. — Enfjineerin'j. 



^rfaiehig* 



SOME BOOKS ON OUR TABLK 



Picfure-maHnff by Photograjihy. By H. P. RoBixsON. 

 (London : Piper & Carter. 1884.) — Many a photographer 

 whose technical manipulation is perfect, whose plates never 

 fog, and the half-tones of whose negatives leave nothing to 

 be desired, fails dismally in the production of anything re- 

 sembling an ariistic picture. That it will not merely 

 suffice to that end to stick up a camera in front of a view or 

 group, and subsequently to expose and develop a plate, Mr. 

 Robinson's excellent manual not only conclusively shows, but 

 points out definitely why, in the plainest and most compre- 

 hensilile manner. His little book should be in the hands 

 of every one who is anxious to preserve the element of 

 beauty in his reproductions of nature, and is not contented 

 with a mere wooden and mechanical copy of the objects he 

 depicts by the aid of the camera. 



The London Water Supply, its Past, Present, and Future. 

 By G. Phillips Bevan, F.S.S. (London : Edward Stan- 

 ford. 1884.) — Beginning with an account of the water 

 supply of mediaeval London from the Thames, the Fleet, 

 the Ty bourne, the Wallbrook, ic, Mr. Bevan carries us 

 down to the existing provision for the metropolis, and gives 

 full details with reference to the various water companies 

 to which we are indebted for it, their plant, sources of 

 supply, and profits ; concluding his book with an account 

 of the various schemes which have been devised for a large 

 increase in the amount of water to be furnished, by bringing 

 it from distant sources, such as Wales and Cumberland. 

 Considering how probably imminent legislation is on the 

 subject of the water supply of London, and how vastly its 

 4,000,000 inhabitants are interested in the question of the 

 purity and economy of such supply, Mr. Bevan's small 

 work should address a very large public indeed. 



2Vie A B C Guide to Physical GfoyrapJii/. (London : 

 Thomas Young, 1884.) — The leading physical features of 

 the earth's surface are set forth in this little book in a way 

 calculated to give a fair rudimentary idea of them. After 

 two perusals of it, we have come to the conclusion that the 

 author means to be funny ; but we speak with some hesi- 

 tation on this point. 



Gas-Burners, Old and Nev3. By Owen Merrimak. 

 (London: Walter King. 1884.) Mr. Merriman gives us 

 a complete history and description of gas-burners, from 

 Murdock's original crude nipple, with its three perforations, 

 through the bat's-wing and fish-tail forms down to the 

 highly complicated and elaborate ones devised by Siemens, 

 Clamond, and others ; and the more simple, but practically 

 equally efficient, burners of Sugg, Bray, and Bronner. 

 The work before us may well tend to dissipate a good deal 

 of unreasoning prejudice against gas-lighting, and to indi- 

 cate how — at all events for domestic purposes — it will hold 

 its own against the electric light for many a long day yet. 



