Aprils, 1876] 



NATURE 



447 



if adopted, would introduce quite as serious, perhaps a more 

 serious ambiguity. Gravity is an acceleration. When we say 

 that gravity is less in a balloon, or in a mine, than at the surface 

 of the earth, or greater at Glasgow than at Manchester, we are 

 speaking of alterations of g — the acceleration due to the earth's 

 attraction ; and it would create confusion to employ this word to 

 designate forces also. " 



Now I do not think that the use of the word gravity as an 

 acceleration is at all common. On the contrary, I have looked 

 into all the books at my command and I cannot find any sup- 

 port whatever for such a use of the word. Every one is accus- 

 tomed to speak of the " force of gravity." To speak of the force 

 of an acceleration would be a complete anomaly. 



All the dictionaries that I have seen support my view that 

 gravity means force, and does not mean acceleration. Take, for 

 example, a good modem book, the Imperial Dictionary. There 

 I find — Gravity (Lat. gravitas from ^az'^j, heavy), i. Weight, 

 heaviness, 2. In philosophy — that force by which bodies tend 

 or are pressed, or drawn towards the centre of the earth — and so 

 on ; showing that gravity always means forca> though it has 

 various shades of meaning in its occasional applications. 



Chambers' Encyclopedia says that the force which causes 

 bodies to fall towards the earth is termed gravity. The article 

 goes on to speak of the force of gravity at the earth, at various 

 heights above the earth, and at the moon. Acceleration is no- 

 where mentioned as a meaning of the word gravity. 



Even if it could be shown that a few people have so far 

 departed from the original etymological sense and prevailing 

 use of the word gravity, as to employ it for an acceleration 

 instead of for a force, this would only prove that the word is, to 

 that limited extent, subject to ambiguity at present. The course 

 that I advocated was to avoid ambiguity by employing the word 

 gravity, used in its most common, and most authoritative sense, 

 instead of a thoroughly ambiguous word weight, in certain cases 

 where misapprehension from the use of the latter word is likely 

 to occur. 



It seems to me that Mr. Stoney and Mr. Walker have been 

 led away by thinking of the letter g, rather than of the important 

 question at i-su3, Tne letter g stands for a number. One way 

 of specifying what number g stands for, is to say that it is the 

 numerical expression for the acceleration due to gravity, that is, 

 the acceleration due to the force of gravity actmg on a falling 

 mass. But this is simply because g is the number which repre- 

 sents the force in Gaussian units on unit mass, and because the 

 force of attraction on any body is proportional to the mass of the 

 body. The latter is an experimentally discovered law. To say 

 with Mr. Walker that " the symbol '^' is gravity," and with 

 Mr. Stoney that "gravity is an acceleration," seem to me expressions 

 equally elliptic on the one hand, and, without full explanation, 

 misleading on the other. 



If Mr. Stoney offers any method of getting rid of the ambiguity 

 better than that already introduced, others will gladly adopt it. 

 As to Mr. Walker's proposal to confine the use of the word 

 "weight" to mean force, my former letter was partly for the 

 purpose of showing this to be impossible. The act of parlia- 

 ment, regulating weights and measures, settles that matter. Mr. 

 Stoney's letter forms, also, a sufficient answer to Mr. Walker's 

 proposal 



University, Glasgow, March 27 J. T. Bottomley 



P.S. — Allow me to thank Mr. Barrett for his information as 

 to the earliest use of spring balances for kilodynes. 



If Mr. Walker is serious in proposing to use vires for British 

 kinetic units of force, he ought to avoid centivires and millirjires 

 for 100 vires and 1,000 vires respectively. These would be utterly 

 incompatible with the use of the prefixes centi, hekto and milli, 

 kilo in the now established metrical system. 



Birds as Astronomical Objects 



The following note which appears in the last number of 

 /ray Feathers (iii. p. 419), seems to deserve more attention 

 rem astronomers than it will perhaps receive unless published 

 where it will meet the eyes of others than Indian ornithologists. 

 I beg leave, therefore, to ask that it may be reproduced in 

 Nature. 



•' Looking at the sim this morning, I saw birds very fre« 

 quently pass the disc. Some were in focus with the sun itself, 

 the wings being quite sharp against the disc, and must have been 

 several miles lu^h, but some were much nearer, and I estimate 



their distance from ms at about two miles by the focus required 

 to see them. These last must, however, have been quite a mile 

 above the earth's surface, and of course many were a great deal 

 higher. 



"I suppose they were Kites, but the appearance there was 

 rather as though the wings were long and narrow like those of 

 Swallows, whereas I should have expected the points to be 

 blunted by the irradiation. 



" The estimated distance between the tips might be a couple 

 of feet. 



" Possibly this may interest some of the readers of Stray 

 Feathers. "J. Tennant, R-E. 



"Roorkee, 23rd September, 1875." 



On this note the editor of Stray Feathtrs, Mr. A. O. Home, 

 remarks : — 



" Many of those birds must have been quite invisible to the 

 naked eye, I have no doubt that Vultures, Kites, and Eagles 

 often soar for hours at heights at which they are ;.aus invisible 

 to us, though we and our doings are '\uite within the grasp of 

 the-j far-seeing gaze. This would help to account for the mar- 

 vellous manner in which, when an animal is killed in the plains, 

 an apparently speckless sky becomes in an incredibly short space 

 of time crowded with ' an heavenly host.' " 



We know so little with respect to the height at which birds 

 do or can fly, that I am sure all ornithologists would gladly avail 

 themselves of any observations on the part of helioscopists or 

 other astronomers that would bear upon the matter, and I may 

 add that perhaps the evidence they could oflfer might be of im- 

 portance as regards the migration of birds. In Mr. Hume's 

 remarks I entirely concur. Alfred Newton 



Magdalene College, Cambridge, March 25 



How Typhoid Fever is Spread 



The case in which the poison of typhoid fever mixed with 

 drinking water was transmitted through nearly a mile of porous 

 earth, and which was mentioned in the abstract of my discourse 

 to the Fellows of the Chemical Society (Nature, vol. xiiL, 

 P- 33i)» is fully described (in German) in the 6th Report of the 

 Rivers' Commission on the Domestic Water Supply of Great 

 Britain. It will shortly appear, in English, in the Monthly Journal 

 of the Chemical Society. Meanwhile perhaps I may be allowed 

 to trespass upon your space with the following remarks : — 

 The outbreak of typhoid fever occurred at the village of Lausen, 

 near Basel, in Switzerland, and it was exhausrively investigated 

 by Dr. A. Hagler of Basel, who has given a full account of it in 

 the " Deutsches Archiv. f^ Klin. Med. xi." The source 

 of the poison was traced to an isolated farmhouse on the 

 opposite side of a ir jgntain ridge, where an imported case 

 of typhoid, followed by two others, occurred shortly before 

 the outbreak. A brook which ran past this house received 

 the dejections of the patients and their linen was washed in it. 

 This brook was employed for the irrigation of some meadows near 

 the farm-house, and the effluent water filtered through the 

 intervening mountain to a spring used in all the houses of 

 Lausen, except six which were supplied with water from private 

 wells. In these six houses no case of fever occurred, but scarcely 

 one of the others escaped- No less than 130 people, or seven- 

 teen per cent, of the whole population, were attacked, besides 

 fourteen children, who received the infection whilst at home for 

 their holidays, and afterwards sickened on their return to schooL 



The passage of water from the irrigated meadows to the spring 

 at Lausen was proved by dissolving in it, at the meadows, 18 

 cwt. of common salt, and then observing the rapid increase of 

 chlorine in the spring water ; but the most important and inte- 

 resting experiment consisted in mixing uniformly with the water 

 50 cwt. of flour, not a trace of which ma-^c its way to the spring, 

 fiius showing that the water was filter e i through the intervening 

 earth and did not pass by an underground channeL 



These are the main features of &e case, but there are other 

 interesting details showing how carefully the investigation was 

 conducted ; for these, however, I must refer Mr. Mitchell Wilson 

 to the works above cited. I*, affords a dear warning of the risk 

 which attends the use, for dietetic purposes, of water to which 

 even so-called purified sewage gains access ; njt withstanding 

 that, as at Lausen, such water may have been used with im- 

 punity for years, until the moment when the sewage became 

 infected with typhoid poison. 



E, Frankiand 



