438 



NATURE 



{March 8, 1888 



octodynets. So, too, Joule's equivalent J, which is about 

 "4-2 X 10^ ergs per gramme-degree Centigrade" (Everett's 

 "Units and Physical Constants"), would be more easily re- 

 membered as 4'2 heptergons. Again, the velocity of light would 

 be (approximately) expressed as 3 octometrons per second, or 

 3 decavelons, if the word vel'vi&re adopted for the unit speed in 

 the C.G.S. system— namely, that of i centimetre (or dimetret) per' 

 second. I have chosen these instances, as cases where the pre- 

 fixes mega- and micro- would be of little use as aids to expression 

 or memory. 



The system 1 am advocating coincides exactly with the 

 method, which I believe most intelligent teachers of arithmetic 

 are adopting, of reckoning the place of any digit of a number 

 by its distance, not from the decimal point, but from the unit's 

 digit. This distance it has been proposed to call the order of 

 the digit, so that the order of the unit's digit is O ; those of the 

 tens, hundreds, &c., i, 2, &c.; and those of tenths, hundredths, 

 &c., -I, - 2, &c. Then, if the ^n/^r of a number be regarded 

 as that of its highest digit, its order is the characteristic of its 

 logarithm. I forbear to dilate on the advantages of this reform 

 in arithmetical language, but it is obvious that the proposed 

 system naturally arises out of it. If the British Association 6r 

 the Physical Society should, after discussion, accept the prin- 

 ciple of the proposed nomenclature, and give it the stamp of 

 their authority, I believe they would add to the benefits they 

 have already conferred on science by the introduction of the 

 C.G.S. system, of units. My proposal would not extend to 

 attempting to replace the words in ordinary use — kilometre, 

 millimetre, kilogramme, &c.— unless they, in the course of time, 

 died out, replaced by the synonyms here proposed on the 

 principle of the " survival of the fittest." 



Harrow, February 27. RoBT. P>. Hayward. 



In Nature of P'ebruary 23 (p. 388) there appears an in- 

 teresting letter from Prof. A. W. Riicker with reference to the 

 equivalent value of the " micromillimetre." It is therein men- 

 tioned that the micromillimetre is commonly employed by 

 biologists as equivalent to one-thousandth of a millimetre ; but 

 that the proper name for the thousandth of a millimetre (^u) is 

 "micrometre," and not "micromillimetre." 



Permit me, however, to suggest that even the denomination 

 "micrometre," may be hardly acceptable to scientific workers. 

 The denomination for the measure of the one- thousandth of a 

 millimetre {fx), or o'oooooi metre, is " micron," and not " micro- 

 metre." 



For the "micron" we have the authority of the " Comile 

 International des Poids et Mesures." One shudders at the 

 thought of tlie confusion likely to arise when computers are 

 required to deal with both micrometre-units and micrometer- 

 divisions. 



Tlie Comite International have a'so recommended the use of 

 the following metric denominations for minute measurements : — ■ 



Denomination. 



Micron 



Microgramme 



Millilitre 



Microlitre 



Symb;- 



7 



ml 



A 



Equivalent. 



o'OOi millimetre. 

 o"ooi milligrame. 

 o 001 litre. 

 O'OOOOOI litre. 



For the millionth of a millimetre we have at present the 

 (C.G.'S.) denomination "micromillimetre" (jU^), as pointed out 

 by Prof Riicker. II. J. Chaney. 



7 Old Palace Yard, Westminster, February 27. 



Allow me to add a few remarks to Prof. A. W. Riicker's 

 letter, published in your issue of February 23 (p. 388). 



Mr. O. J. Broch, Correspondent of our Institute in its Section 

 6f Mechanics, and Director of the International Board of Weights 

 and Measures, having kindly undertaken to ascertain by actual 

 measurement my pendulum's coefficient of expansion by heat, 

 began by asking how old it was. On my expressing surprise at 

 such a question, he told me that, having carefully measured the 

 length of a brass rod recently made and i metre long, he found 

 that it became shorter by 8 micions in the first year, and 3 more 

 in the second one. Micron is currently used here to express 

 I /icoo of a millimetre. French botanists call it /u, and seldom 

 use its first decimal because they cannot see such a small space. 



The only objection against micron is that, unlike other sub- 

 divisions of the metre, it does not define its length by its name. 



But the word metre has itself the same fault. It is the ten- 

 millionth part of a quarter meridian, and is, according to 

 Clarke's computations, too short by o'2 millimetre, or, more 

 exactly, 1877 microns. Improvements in geodesy will probably 

 alter in either sense that fraction which is too small to disturb 

 terrestrial requirements. 



The quarter meridian being the true basis of our metrical 

 system, it ought to have a name of its own, and might be called 

 megist, as being the greatest space accurately measured. It 

 should be the metre used in astronomy. Thus the velocity of light 

 would be 30 megists, the motion of the star Aldebaran in the 

 line of sight would be 18 megists per hour, and the sun's distance 

 15,000 megists ± 50. To give the latter in kilometres Or miles 

 is tantamount to describing the height of St. Paul's in London 

 as being 1,100,000,000 microns. It is useless to express a 

 distance in units so small that one of them may be added or sub- 

 tracted without altering our useful notion of the whole sum. 

 Moreover, those who can grasp at once a practical idea of such 

 huge numbers are few and far between. 



February 29. Antoine d'Abbadie. 



Coral Formations. 



Mr. G. C. Bourne's observations, as far as described in last 

 week's Nature (p. 414), appear to corroborate fully the view 

 that corals grow more rapidly and luxuriantly on those parts of 

 a reef or bank where there is an abundant supply of food, and 

 only in scattered patches where the food supply is limited or 

 where there is a quantity of sand or other inorganic materials in 

 the currents. He states his belief that "the favourable condi- 

 tions are due to the action of currents on coral growth." If it 

 be not the food in these great oceanic currents, then Mr. Bourne 

 should tell us what it is in "the action of currents " bathing the 

 outer slopes of a reef that renders them favourable to growth j 

 does he hold to the old view of more oxygen in the water ? 



It is to be hoped that Mr. Bourne has observed some of the 

 corals feeding on the outer slope or in the lagoon, and can tell 

 us of what their food consists. It will be interesting to know if 

 he has worked his tow-nets in the outer currents, in the " strong 

 currents," and in the still water, and has made a comparison of 

 the results. If he has done so, his paper will doubtless be one 

 of great interest and value. 



There would appear to be a slip of the pen in the passage 

 where Mr, Bourne i^efers to a current impinging directly on a 

 slope. John Murray. 



An Incorrect Footnote and its Consequences. 



Thanks to the wide circulation of Nature, my original note 

 with the above heading has attracted attention in quite a number 

 of the proper quarters. Several letters have reached me on the 

 subjdct, and more than one of the writers, after reporting that 

 the Dcnicnstratio eliminationis Craincriaiiic had been found 

 properly catalogued under De Prasse, proceed in consequence to 

 express their surprise at Baltzer's mistake. Mr. Copeland's 

 letter in yesterday's Nature (p. 343) adds another instance of this 

 correctness of cataloguing. The additional fact, which he men- 

 tions, that there are two copies of the original edition of the 

 Dcmonstratio in the Dun Echt library is very interesting, and 

 is a fresh proof of the existence there of valuable rarities. 



When, however, Mr. Copeland diverges into the fruitless path 

 of " the might have been "he is much less pleasantly instructive. 

 Having read my letter on the search for a work by MoUweide,. 

 and on the discovery that the work meant was not by Mollweide at 

 all but by De Prasse, Mr. Copeland turns to his catalogue under 

 Mollweide, finds a cross-reference to De Prasse, looks up De 

 Prasse, picks out the desired plum, and is pleased accordingly. 

 In all this there is nothing singularly lucky or otherwise : it is 

 exactly what ought to have happened. Mr. Copeland ap- 

 parently thinks that the cross-reference in the Dun Echt 

 catalogue was the missing link ; but if he had had occasion to 

 look up other catalogues besides his own he would have found 

 the same cross-reference or a more complete one, and might then 

 have given my helpers in the search a little more credit. The 

 fact is that the booklet of mathematical tables which was the 

 cause of the cross-reference (and whose title Mr. Copeland care- 

 fully transcribes) is a comparatively common book, having gone 

 in its time through several editions. Its name is thus of not 

 infrequent occurrence in catalogues, being placed under De Prasse 

 with a reference to Mollweide, or vice versa ; and, so catalogued^ 



