Jan. 3, 18S9] 



NA TURE 



22 



Rariorum Scaniae item Catalogus Plant arum Rariorum 

 Smolandije" (1728), in the possession of the De Geer 

 family (Leufsta Library); and by " Spolia Botanica " 

 1729), the original of which is in the possession of the 

 Linnean Society, and is considered to have been finished 

 unvards the end of 1729. This seems, however, improb- 

 ;ible, the date of dedication (to Prof. Roberg, one of 

 Linnaeus's teachers at Upsala) being May 5, 1729. The 

 work is accompanied by twelve facsimile drawings of the 

 principal representatives of the Lapland flora. 



This part of the first series contains copious and 

 explanatory notes by the late Dr. Ahrling, a work which 

 must have entailed very great labour. After his death, 

 his editorial duties were undertaken by Dr. M. B. 

 J*rederus. The second series will be edited by Prof. G. 

 Strom. 



irst Principles of Physiography. By John Douglas' 



(London : Chapman and Hall, 1889.) 



IE ever-increasing number of te.xt-books on this 

 ibject is evidence that the study of physiography is 



lining in popularity. The object of the book before us, 



the author states in his " Prologue on the Beach," is to 



ive a systematic statement of the nature of the forces at 



irork in the world, and of the changes which the matter of 



the world undergoes. The book is obviously designed to 



.cover the syllabus issued by the authorities at South 



'TCensington, although no mention of this fact is made. 



The first part of the book deals with force, but for some 

 reason or other, force is not defined until p. 26, and there 

 only in an obscure place. The author's notion of treating 

 elementary chemical ideas is somewhat peculiar ; to 

 make statements about positive and negative elements 

 without explaining the meanings of those terms, and to 

 use formulas like NH3 and H2SO4 (p. 36) without naming 

 the compounds they represent, is scarcely the way to 

 inspire a student with confidence in his teacher. 

 ■ No less than 23 pages are devoted to tables, all of more 

 or less interest to students of physiography. 



Perhaps the chief novelty of the book is the introduction 

 of copious quotations from, and references to, standard 

 works. Their introduction as footnotes, however, is 

 rather objectionable, as it tends to discontinuity. A good 

 deal of information is undoubtedly given, but the style is 

 not such as to commend it to those who are just com- 

 mencing the study of science, and these, it must be 

 remembered, constitute the majority of those who take 

 up the subject of physiography. 



LETTERS TO THE EDITOR, 



\The Editor does not hold himself responsible for opinions 

 expressed by his correspondents. Neither can he under- 

 tc^e to return, or to correspond with the writers of, 

 rejected manuscripts intended for this or any other part 

 of Nature. No notice is taken of anonymous communi- 

 cations.] 



"Enginters" versus "Professors and College Men." 

 Prof. Greenhill is, himself, one of many proofs that the 

 dJMinction between '* Engineers" and " Professors and College 

 Men " is a Cross Divi ion. Every "Engineer" ought to be a 

 "highly-trained College man." If he were, he would know 

 at once, from the very first sentence of the Principia {Quantitas 

 materia est mensura ejusdein &c , &c.) that mass is the personal 

 p. operty of a body, one of the invariable things in nature : — 

 and not an accidental property dependent, for its amount and 

 even for its very existence, on the momei.lary surroundings. 

 The letter M has hitherto been used by Newtonians in 

 this sense. If anyone has since attached to it another 

 and different sense, he is responsible for the consequent con- 

 fusion. Would it not be well if Prof. Greenhill, and the School 

 to which he has attached himself, would kindly leave to New- 

 I tonians their M, as defined for them by their Master ; and (with 



severely logical consistency) turn it upside down (thus, W) when 

 they wish to embody their own revolutionary definition? No 

 Newtonian will refuse to recognize V^v'lig as a correct expres- 

 sion for so much energy: — though he will probably think it 

 both clumsy and complex, and will prefer to write as usual his 



Uv-l2. 



I am curious to know how Prof. Greenhill would deal with 

 physical Astronomy. What is his measure of the earth's mass ? 

 According to the analogy of his "'units ofg pounds " the earth's 

 mass is at present (near perihelion) to be spoken of as if it were 

 some 6 or 7 per cent, greater than it was six months ago ! 



The whole of this attempt to improve on Newton is caused 

 by unwillingness to face, once for all, the small amount of labour 

 and thought requisite for learning or teaching how to pass frota 

 one system of units to another. A properly taught student 

 learns, very early in his career, that this is no awful and myste- 

 rious process : — in fact that it is, throughoat, quite as simple 

 in principle as is the passing from miles per hour to feet per 

 second. 



And I venture to assert that such a student would attack with 

 ease and confidence any fair question (i.e. one free from mere 

 tricks or traps) connected with the subject. This one, for 

 intance : — 



" How many of the following quantities (taken in order) can, 

 by selection of the requisite .system of units, be simultaneously 

 expressed by one and the same number. First, when that 

 number is given ? second, when it is not ? 



(a) The weight of a ton, at sea-levcl, at the equator. 



(b) The speed of light in vacuo. 



(c) The average kinetic energy of a particle of hydrogen at 

 o°C. 



(d) The minimum compressibility of water at low pressures. 



{e) The mean angular velocity of the earth about the sun. 



Express the requisite units in C.G.S. measure, when the com- 

 mon numerical value, above mentioned, is log,T ; and also when 

 it is not assigned." 



Of course it is understood, and this is my answer to Prof. 

 Greenhill's first question, that the student 70ould be furnished 

 with all the necessary data, experimental or otherwise, expressed 

 in definite assigned units. 



In answer to Prof. Greenhill's second question I need only 

 say that it is no part of my case to assert that all statements, 

 made by "College men," are necessarily characterized by 

 definiteness, by accuracy, or even by common-sense. 



December 21, t888. P. G. Tait. 



The Sun-spot Cycle. 



It may interest some of the readers of Nature to learn thart 

 an expected change has just been observed upon the solar 

 surface. 



It is a well-established fact that in each new series of sun- 

 spots the first spots of the cycle are seen in high solar latitudes, 

 and that as the number of spots increases there is a commoa 

 drift towards the sun's equator, the spot area becoming most ex- 

 tensive as the sixteenth parallel of heliographic latitude is reached 

 During late yeas the spots have been diminishing in number 

 and size, and approaching the solar equator ; and in the past 

 twelve months very few spots have been seen on the sun's sur- 

 face, and all in low latitudes, that recorded on December 21 

 being 4° south of the solar equator. The close of the year has, 

 however, witnessed a change, as a small spot is recorded on the 

 Stonyhurst drawing of December 30 at 36° south latitude. Spots 

 near the equator will probably continue to be observed for some 

 time, but, whilst they are diminishing, those in higher latitudes 

 will be on the increase. S.J. Perry. 



Stonyhurst Observatory, Lancashire, December 31, 1888. 



"Renaissance of British Mineralogy." 



Mr. Fletcher's admirable address on a " Renaissance of 

 British Mineralogy," of which a report was published in a 

 recent issue of your paper, calls timely attention to the present 

 condition of the science. Mineralogy as a popular study seems 

 dead : the chemists have deserted it for a study of complex 

 organic compounds, so that it has become a mere hanger-on of 

 geology. The science is now not thoroughly taught in any in- 

 stitution in this country, and teachers therefore have no means 

 of acquiring knowledge, in the only really useful way, by working 

 under the acknowledged masters. This is especially the case wi>b 



