154 



NA TURE 



[December 13, 1894 



be effet lively dealt with in the limits of an ordinary letter. I 

 propose therefore to deal with the subject in a special article to 

 be written shortly. A. E. H. LoVE. 



St. John's College, Cambridge, December 3. 



Gravitation. 



May I asit Dr. Joly whether Newton himself did not point 

 out that a graduated tension excited by matter in a continuous 

 inextensible medium, of an intensity proportional to the mass 

 and the inverse distance, would account for gravitation ; and 

 whether he did not refrain from furiher elaborating this idea 

 because there seemed at that time no adequate way of explain- 

 ing the existence of such a tension? Oliver J. Lodge. 



" Outlines of Quaternions." 



Mav I make a short explanation on one or two points on which 

 my reviewer (Nature, November 22) does not appear to have 

 understood me ? 



(1) I mentioned Prof. Hardy and Dr. Odsticil's names in one 

 or two places, because I quoted their language verbally. I found 

 it was better than any language I could devise. 



(2) The extraordinary oversight on p. 76 would never have 

 seen the light had I had either good health, or the assistance of 

 a friend, in correcting the proofs of a MS. which was written 

 at odds and ends of time, at places as widely separated as 

 Norway, Gibraltar, and India. 



(3) My reviewer says eq. S of p. 40 — :' = \' — I — "plays 

 sad havoc with one's very definitions." Having defined i as a 

 right versor on p. 37, and explained on p. 39 that «- means ii, 

 I deduced in the usual manner the eq. — i^ — - I, or — 



it = r- = - I = V - I n' - I. 

 Hence I concluded that « = V - i ; and I fail to see how I 

 have played havoc with my definitions in doing so. Had I 

 begun by explaining that Hamilton built up hissystemby treating 

 \ - I as a right versor perpendicular to the line it operates on, 

 I might have been open to criticism ; but I did not do so. I 

 took another course, which may not have been the best one ; 

 but that is a different thing from violating one's own definitions, 



H. W. L. Hime. 

 24 Haymarket, S.W., November 26. 



[A pointed reference to a scientific writer usually implies 

 one of three things — that the writer is an authority on the 

 particular subject under discussion, or has made a noted dis- 

 covery in connection with it, or has been guilty of a serious 

 blunder. Dr. Odstrcil's corollary hardly comes under the second 

 category, and Prof. Hardy's statement differs in no essential 

 word from Hamillon'sown language in the " Lectures " (p. S3). 



Equation 8 (p. 40, in "The Outlines of Qualernions ") asserts 

 the equality of J, y, /■, - i, - /, - /!•, and as these symbols are 

 by definition all different, the said equation is inconsistent with 

 the definitions. To speak of v' - I as an indeterminate right 

 versor, that is, an operator which rotates any vector through a 

 right angle about an indeterminate axis — a most difTicult 

 operation for the mind even to imagine — may be permitted as a 

 figure of speech ; but to equate this backboneless thing to a 

 real unit vector or right versor with all its powers of action, is 

 making a serious demand upon the credulity of the student. 

 After titfinirig i, j\ /• as symbols involving both axis and angle, 

 what right or reason has Colonel Hime thus arbitrarily to 

 annihilate the axis ? Is it not playing havoc with the very props 

 of the calculus? — The Reviewer.] 



THE WARBLE FLY} 



TT is only within comparatively recent years that much 

 *■ attention has been paid to the insect pests of the 

 farm and garden. It is true that when these assume un- 

 usuilly devastating proportions, especially when they 

 make their appearance suddenly, as in the case of locust- 

 swarms, the attention of whole nations is called to them 



' " ObMrvationi on Warble Fly or Ox Bot Fly {ffy/'oJtrma I't.'U, Dc 

 Gmt)." By Eleanor A. Ormirod, F. R.Mct.Soc., &c. (tendon : Simpkin, 

 Marvhall, Hamilton, Kent, aorl Co , Ltd., 1S94.} 



MO. 131 I, VOL. 51] 



for the moment ; but the loss caused by less obtrusive 

 creatures may proceed unchecked and almost unsuspected 

 for years, without attracting the notice even of those who 

 suflfer from it most. But there are now many entomolo- 

 gists, among whom Miss Ormerod deserves special notice 

 in England, and Prof. Riley in America, who have been 

 working zealously for years to diminish the loss and 

 injury caused by injurious insects ; and the pamphlet 

 before us, with its clear descriptions and statistics, and 

 excellent illustrations, conveys a mass of information, in 

 a very handy form, which certainly deserves the most 

 serious attention of all who are interested in the cattle 

 and leather trades, whether as graziers, butchers, or 

 tanners. 



The total loss caused by the warble fly in the United 

 Kingdom alone is estimated at something like /8,ooo,ooo 

 per annum ; an enormous amount, but which the facts 

 given in Miss Ormerod's pamphlet fully appear to 

 bear out. When hides are sometimes so deteri- 

 orated that the loss on each may be as much as 

 from twenty-five to thirty shillings, to say nothing 

 of hides rendered utterly worthless ; cattle killed, 

 or the best parts of the carcase destroyed, and 

 diminished yield of milk, the importance of the matter 

 becomes very apparent. And beyond this, there remains 

 a very serious question which Miss Ormerod has not 

 touched upon at all : how far the milk of badly-infested 

 cows, or the apparently sound portions of a carcase, even 

 when all the obviously diseased part has been con- 

 scientiously removed, may be liable to cause disease 

 in man— disease, possibly, of a nature the origin of which 

 is at present absolutely unknown and unsuspected by 

 medical men. And yet we remember once to have met 

 with the statement that the best hides generally contained 

 warbles. This, however, if true in any sense, could 

 only mean that the fly attacks the strongest and healthiest 

 animals in preference to weaker ones, thereby of course 

 increasing the mischief produced by its attacks. 



Although the insect is so abundant that as many as 

 500 maggots have been found in .a single hide, yet the 

 fly is rarely seen. When the cattle are attacked by it, they 

 gallop wildly about, with their tails in the air, and seek 

 the shelter of trees or sheds, or rush into the water ; and 

 in any of these situations, the tly docs not appear to 

 follow them. Cattle will act in the same manner when 

 attacked by true gad-tlies, one of the largest British 

 species of which, Tabaniis boviniis, is likewise noticed 

 and figured by Miss Ormerod in her pamphlet. The 

 gad-flies, however, simply pierce the i-kin of the cattle, 

 and suck their blood, but inflict no permanent injury; 

 and their larvae are subterranean, and not epizootic. 



According to the observations of Prof. Riley in 

 America, the egg of the warble fly is deposited on, 

 and not under, the skin. In the earliest stage of 

 the maggot, which Miss Ormerod has herself ob- 

 served, it is a small blood red worm-like creature, 

 scarcely visible to the naked eye, embedded in a 

 slight swelling, composed of blood-red tissue, through 

 which a fine channel, no wider than a hair, passes up 

 to the surface of the skin (Fig. i). In the very young 

 stage, the maggot, which ahvaysrests with its head at the 

 bottom of the sore, and the breathing apparatus, which is 

 at the opposite extremity of the body, directed towards 

 the opening which communicates with the external air, 

 is provided with two forks or diggers, probably used for 

 piercing through the substance of the hitje. Inthisstage, 

 too, the maggots are capable of inflating theinselves with 

 fluid which they have apparently no means of discharg- 

 ing, and become so hard that they can scarcely be 

 compressed with the fingers, thus forming living and 

 growing plugs, which act the part of setons, and which 

 cannot be pressed back out of the wound, more especially 

 as they arc furnished with short bands of prickles along 

 a portion of the back. Having penetrated the hide, the 



