September io, 1891] 



NATURE 



445 



strokes. These precepts and a fewminor ones have been easily 

 followed in all cases. I inclose a few lines copied from your 

 interesting journal by a youth who does not understand English : 

 he would have done this work with more care had he known 

 that I wanted merely a sample. At all events it is most easy 

 to read. 



Straight letters without hair lines give the reader a comfortable 

 facility which is a far greater compliment to a correspondent than 

 the "r/mrSir" imported from England to France during the 

 last fifty years. We suppose that slant writing has been 

 invented on your side of the Channel, and we call it therefore 

 ecriture aitglaise. However, experience seems to show that it is 

 more easily deformed than a straight one, and that it degenerates 

 often into an illegible scrawl, causing much loss of time, or even, 

 what is worse, a tiresome amount of perplexity and worry. We 

 are told that the schoolmaster is abroad, but I am afraid that 

 he leads our children on a false trail far away from the main 

 aim of writing, which is legibility. Is the invention of type- 

 writers the antidote or the outcome of illegible slants ? Some 

 of your philosophers may answer this question while giving a 

 wholesome lesson to the schoolmaster. 



A. d'Abbadie (de I'lnstitut). 



Abbadia, Hendaye, France, August i6. 



Cordylophora lacustris. 



In Nature for June 4 (p. 106) Mr. John Bidgood recorded 

 the presence of this Hydrozoon in vast numbers on submerged 

 roots and stems in the Ant, Bure, and Thurne. Till then its 

 only known Norfolk locality was that given in Allman — "an 

 agricultural drain near Lynn Regis." This summer innumerable 

 colonies were to be seen on weed floating on the surface on both 

 sides of the Thurne from Ludham Bridge right up to Hickling 

 Broad. A boatman told me he had seen " them insecs" every 

 summer for many years past. Mr. Edward Corder, the Secre- 

 tary of the Norwich Natural History Society, took some early in 

 June, and some, which he was good enough to send me, is still 

 living in a 4-ounce bottle. All the authorities state that Cor- 

 dylophora is a "light-shunning animal," and thelocalitieshitherto 

 recorded certainly warranted such a conclusion. But the 

 colonies taken from the surface of the water by Mr. Corder, and 

 those I took some time later, were stronger and cleaner than 

 those obtained from below the surface. I distributed some 

 of the gathering which I brought back to London, and learn 

 that it is all doing well in ordinary aquaria. Some that I sent 

 to Mr. Bolton for distribution unfortunately died in transit. One 

 large colony, some eight inches long, on the stem of a Potamo- 

 geton, was kept in the shade for a fortnight ; the tubes became 

 flaccid, and the hydranths pendent, but they revived within 

 twenty-four hours when exposed on the ledge of a window with 

 a western aspect. This seems to point to a change of habit. 

 All the colonies were doubtless founded below the surface of 

 the water, and the weeds, when cut to clear the fairway for 

 wherries, were floated up towards Hickling Broad by the tide. 

 But if reproduction takes place — as it certainly does — under 

 these conditions, is it not probable that we shall have a race 

 tolerant of direct light, if not as sensitive thereto as Hydra 

 VZtlgaris? HENRY SCHERREN. 



5 Osborne Road, Stroud Green, N., September 3. 



Absolute and Gravitation Systems. 



The present condition of things is such that students of engi- 

 neering need familiarity with, and ability to use, both systems o 

 measuring force and related quantities. It seems necessary, 

 therefore, that the transition from one system to the other 

 should be kept clear of complications, and be presented as the 

 simple matter which it really is. But in two text-books which 

 have come to my notice, each offering points of excellence, and 

 both evidently written by competent hands, a change in the 

 unit of tnass occurs in passing from the absolute to the gravita- 

 tion system. The unit-mass is defined as the mass in which 

 unit-acceleration is produced by unit force, which, of course, 

 gives about 32 pounds as the mass-unit for the British gravita- 

 tion system. 



There is, in my opinion, much that is undesirable about this 

 method of statement ; the new mass-unit appears quite arti- 

 ficially in this one only of the many uses of the conception of 

 mass, for the purpose, I suppose, of making it possible to put in j 



NO. I 141, VOL. 44] 



generally applicable form such statements as : " Force is mea- 

 sured by change per second in momentum." My particular 

 objection to it, however is that it locates the point of divei^ence 

 among the fundamental units instead of among those derived 

 from them. Does it not seem preferable to begin with units of 

 mass, length, and time ; to construct derived units, and to make 

 common use of these as far as possible, postponing the differ- 

 entiation of the two systems till the moment when it actually 

 occurs ? Surely it has been pointed out often, since the days of 

 early exposition of these matters by Maxwell, Tait, and others, 

 that the force-unit is the first cardinal point of difference, and 

 that the absolute system simplifies here, while the gravitation 

 system adopts another convention, which may be called arbitrary 

 as opposed to the simpler one fixed upon by its rival. 



In the hope of hastening the day of agreement in presenting 

 the connection of ideas which underHe so much of modern 

 physics and its applications, I have thought it permissible to 

 state in summary, and for British units, the scheme used in my 

 own teaching of mechanics. The claim is not advanced that 

 the numerical work becomes different ; indeed, the appended 

 table is equally valid whichever basis be chosen ; but there does 

 seem to be a gain in logical clearness, as well as in what we 

 may call historical accuracy. 



Absolute System. — Fundamental units : foot, pound, second. 

 Units of force, work, impulse derived in the usual way, so as to 

 make proportional factors unity. 



Gravitation System. — Fundamental units as before. Unit of 

 force, the weight of one pound under circumstances specified to 

 the required degree for scientific definiteness (locality, vacuum). 

 Units of work and impulse connected with the force-unit, so as 

 to make proportional factors unity. 



The table shows the matter at a glance, gy is the value of ^ 

 for the standard circumstances, and is to be regarded as a divisor 

 in each case affecting the product of the other factors. The 

 other symbols explain themselves. 



Absolute. 

 P = mp, 

 (work) {Yds = (change in) -— , 



(impulse) / Fdt = (change in)mv. 



Gravitation. 

 p ^ »'/ 



(work) / Yds = (change in) "^ , 

 J 2gi 



mpulse) / Pdt = (change i 



in) 



^x 



The choice of force-unit here affects what is logically subse- 

 quent to it, as it must ; but leaves unaffected what is logically 

 antecedent, as it ought. 



So small a change as that of regarding g-^ as a divisor of m 

 alone changes the basis of presentation ; but there is an important 

 difference of thought involved. Frederick Slate, 



University of California. 



Eucalyptus as a Disinfectant. 



In a paragraph on the use of Eucalyptus branches for disin- 

 fection, as recommended by Baron von Mueller, you have un- 

 intentionally stated that to be the manner in which I have used 

 Eucalyptus. 



For the last two years I have used "Tucker's Eucalyptus Dis- 

 infectant" (a solution of antiseptics in the essential oil) in all 

 cases of scarlet fever and diphtheria, and have not had one case 

 of infection. In the former disease I have not used any isola- 

 tion, and in most cases have not excluded the other children of 

 the family from the sick-room. None of the cases, except two or 

 three that were severe, were kept to their bed-room more than 

 ten days ; the isolation of six or eight weeks being unnecessary, 

 as the cuticle is perfectly disinfected. This is accomplished by 

 rubbing the disinfectant over the whole body twice and then 

 once a day for ten days. 



Baron von Mueller, in a letter I received from him, quite 

 approves of my method of disinfecting by inunction. I read ^ 



