April 3, 1885.] 



♦ KNOWLEDGE ♦ 



291 



force, which is required to produce it. The statical force of gravity 

 is measured by weight, and it produces an acceleration g. If, 

 then, K be another statical force which produces an acceleration /, 

 we hare 



Now in Atwood's arrangement, TTis the weight of the descending 

 mass, but the whole statical force acting on it is 11'— T, where T is 

 the tension of the cord. 



TTT m 



Hence the acceleration, (■= -I in (I). 



W - ^ ' 



Again, if IF' be the weight of the ascemling mass. 



/= 



T-W 



equating the two values of /, we get 

 •,p_ 2irTF' 



w + ir' 



which, substituted in equation (1), gives us 



• w+ II-'-' 



All this is carefully and clearly explained by the authors in 

 question. J. R. Campbell. 



THE DEY-EAETH SYSTEIT. 



Christchurch District Drainage Board 

 (Engineer's Department), 

 Christchurch, Xew Zealand, Feb. 5, 1S85. 



[1660]— The article on " The International Health Exhibition," 

 in Knowledge of Oct. 31 ult., would lead one to suppose its author 

 recommended that excrementitious matter should be kept out of 

 the sewers, and the ash-closet adopted, in the "suburbs first, and 

 eventually in the metropolis. I wUl, with youi* kind permission, 

 say a word or two against such a proposal. 



In small towns, sewers or no sowers may be a, debatable ques- 

 tion ; bat large centres must, if they wish to keep themselves in a 

 decently healthy condition, be served by a system of sewers designed 

 and constructed on proper and scientific principles. 



I may state, in parenthesis, that it is only of comparatively late 

 years the London sewers could be properly so de.signated, and, at 

 the present time, I believe, many of the provincial EnglLsh towns 

 possess the luxury of the old-fashioned and nowadays universally 

 condemned, sewers, which are not self-cleansing. Of such it is not 

 my intention to speak further than to say that the}' are worse than 

 useless, being little better than elongated cesspits. 



In densely populated towns it is simply impossible to satisfac- 

 torily get rid of the ordinary household sewage without sewers ; 

 aud, this being granted, it is the greatest mistake to exclude 

 facal matter. It has been abundantly proved by the highest 

 authorities in chemistry that the objectionable features, and even 

 the chemical constituents of sewage are not appreciably altered by 

 allowing the water-closets to be connected. Therefore, if we are 

 to have sewers at all, let us make the most of them, and let them 

 carry all sewage matter that will safely travel through them. 



The dry earth system is an excellent one, where it is properly 

 aud systematically attended to ; but I firmly believe, and am borne 

 out in my opinion by men who have made the matter a special 

 study, that this very rarely occurs, and then only in cases of 

 isolated dwellings, where servants are specially told off for the 

 purpose. At a moderate estimate it would take the enormous 

 quantity of 2,000 tons of dry earth per diem to serve London and 

 the suburbs, and I would ask how could such a quantity be regu- 

 larly supplied, especially in winter ; and, even wore this objection 

 overcome, who is to see that the earth is properly applied ? 

 Sifted ashes may certainly be made to do duty, but the supply 

 would be quite inadequate, more particularly in summer, when they 

 are most required. 



The author referred to goes on to say that " it can be clearly 

 shown they (the excrementitious matters) may be turned to pro- 

 fitable account." This is by no means borne out by facts, the 

 result of such experiments being rather the other way ; for I need 

 scarcely say that the cost of removing the w.c. products of London 

 by scavengers would be more than the ratepayers would care to 

 incur ; besides which, the demand for such products is extremely 

 limited. 



This city and district has recently been sewered on what is known 

 as the separate system — i.e., the rain-water is excluded ; water- 

 closets are being freely adopted ; the sewers are regularly and ys- 

 tematioally flashed ; there is no sj'mptom of any nuisance at the 



street ventilators, and the cost of disposal will shortly bo covered 

 by the sale of the produce from tho sewage-farm. 



In conclusion, I will only add that house-connections must bo 

 made on approved principles aud imdor rigid inspection, and if this 

 is done no danger need be apprehended. 



ElPWAlUl CuTHBEliT, SI. luSt. C.E. 



LETTERS RECEIVED AND SUORT ANSWERS. 



PfzzLEU. Does it follow, because a thing "passes away," that 

 hence it is annihilated .' Your own letter has left you for ever and 

 ever, but it lies before mo as 1 write. Besides, the conservation 

 of energy may bo rigidly true in a mechanical sense, and yet, as 

 far as usefulness or fitness for living creatures in concerned, a cos- 

 mical deterioration may bo going on. — Uolmhale. There ia no 

 "difference in the treatment of light from the sun and that from a 

 fixed star." Tho sun we are regarding is the sun of eight and a- 

 half minutes ago, whether ho be on the horizon or at his nearest to 

 the zenith. If we suppose our atmosphere to bo annihilated (to 

 get rid of the effect of refraction), tho moment our horizon was 

 interposed between us and the undulations of tho aether impinging 

 on it — t.t'., those which started from tho sun eight and a-half 

 minutes previously — he would disappear. Tlio light waves would 

 continue to arrive ; but meanwhile the earth's rotation wonid be 

 interposing more and more of her solid substance between our eyes 

 and tho source of those waves ; in other words, raising the horizon 

 higher and higher, or depressing the sun more and more. Quite 

 obviously, when the earth has turned on her axis sufficiently to 

 depress tho eastern horizon of the observer below the wave-fronts 

 of the light which left the sun eight and a-half minutes before he 

 will seem to rise. — H. Trueman Wood. Received with thanks. — 

 W. J. C. It is simply impossible to explain tho question in a 

 popular manner. Thirty years ago, Routlcdge & Co. published a 

 little book on "Algebra and Plane Trigonometry," by Professor 

 J. R. Young, in which, if you can get hold of it, you will find an 

 effective trigonometrical demonstration that the decimals in tt are 

 interminable. Of course, the circumference is expressed with more 

 than sufficient practical exactitude by a comparatively few decimals. 

 To take \"our own proportion, 113 : 3<J5, this will make your circum- 

 ference some '00000027 (twenty-seven hundred milliouths!) too 

 great, a quantity it might well puzzle you to measure in a circle 

 sixty inches in diameter. — C. A. M. A few very elementary con- 

 siderations will show you that your query is unanswerable. Im- 

 primis, the distance of the nearest star in Hercules is, in the 

 present state of our knowledge, immeasurable and practically 

 infinite ; and the components of that constellation — or many of 

 them — may well be as far off again ! Suppose, though, that we 

 were moving at any given rate towards some star at a known dis- 

 tance, a division sum would enable us to find when we should reach 

 it. For example, if the sun were travelling straight to Arcturns, of 

 which the parallax is 013", at the speed you specify, we should 

 arrive there in 1,580,033 years. — Thomas Avers. It is the power 

 of transmitting heat which was first called diathermancy by 

 Melloni : diathermancy bearing tho same relation to radiant heat 

 that transparency does to light. I cannot conceive what you mean 

 by " a heat-ray being reflected on itself." Such heat as is not 

 transmitted is absorbed and converted into motion of the trans- 

 mitting body — i.e., it heats that body itself. You are under some 

 delusion as to any " insult" having been intended. Tho gentleman 

 to whom yon refer is thousands of miles away, and has never set 

 eyes upon one of your letters. Hence liia memory was never 

 appealed to at all. — E. L. G. No one appreciates your valuable 

 scientific contributions to these pages better than I do, but you 

 really must take your " sea " that "fell from tho sky, about fifty 

 centuries ago," to the Geological Society at Burlington House, for 

 criticism by experts ; because, merely as a matter of fact, it never 

 fell on the spot whence I write. — T. S. Chrisiophek. Pardon me 

 for pointing out that, in strictness, your question has no meaning, 

 since the apparent passage of tho sun from the north to the south 

 of the equator has its origin merely in the tilting of the earth's axis 

 23^ 27' from a perpendicular to her orbit, the ecliptic. If, though, 

 we suppose that the earth is fixed, aud that the sun annually goes 

 round her inside of a sphere whose radius = that of the earth's orbit, 

 then would he be 37,803, .500 miles north of the earth's equator 

 (produced to such sphere) on June 21, and the same distance south 

 on Dec. 21. — J. E. S. "Earthquakes" just un trap peu lard. The 

 other, as yon will see, inserted. — W. Eu.ssell. Your " very bright 

 star " is only the planet Saturn — that's all. See " The Face of the 

 Sky, which appears here every fortnight. The only other planet 

 fairly . aible to the naked eye at present is Jupiter. You will find 

 him just to the north-west of ilogulus, a Leonis (" The Stars in 

 their Seasons," Map IV.). I am unacquainted with any fuUer list 



