September 24, 1891] 



NATURE 



493 



of Livingstone's services. The strictly biographical part 

 of the work is equally well done. All the world agrees 

 that Livingstone was one of the noblest men who have 

 ever devoted themselves to travel. This is felt strongly by 

 Mr. Johnston, and he has been able to express his feeling 

 effectively without extravagance and without any attempt 

 at fine writing. The book will especially interest young 

 readers, but may be studied with pleasure and profit by 

 readers of any age. There are many good illustrations 

 from photographs or drawings by the author, and seven 

 maps by Mr. E. G. Ravenstein. 



LETTERS TO THE EDITOR. 



{The Editor aoes not hold himself responsible for opinions ex- 

 pressed by his correspondents. Ntither can he undertake 

 to return, or to correspond with the writers of, rejected 

 manuscripts intended for this or any other part of l^ATUKV.. 

 No notice is taken of anonymous communications. \ 



The National Home-Reading Union. 



When one remembers the difficulties with which one's own 

 first efforts to study Nature were beset, it seems a pity that any 

 youthful student should be ignorant of the existence of an 

 organization which can do much towards making his path 

 smooth. 



The National Home-Reading Union endeavours to guide 

 those who cannot obtain aural instruction into the safest and 

 most attractive roads. Lists of books are drawn up ; difficulties 

 and discrepancies in systematic reading are, as far as possible, 

 foreseen and removed in the pages of the magazine ; questions 

 are answered by those who conduct the courses. Last year and 

 the year before, the courses on organic and inorganic Nature 

 were in the charge of Mr. Francis Darwin, Dr. Hickson, and 

 Dr. Kimmins. This year, geology is undertaken by Mr. Marr, 

 and cryptogamic botany by Mr. Murray ; and any persons who 

 wish to work at these subjects may save themselves much labour 

 and misplaced reading by writing to the Secretary of the Union, 

 Surrey House, Victoria Embankment, for a prospectus. Mr. 

 Murray tells me that it is often pitiful to see how mnch effort 

 has been wasted by people who come to the British Museum to 

 educate themselves, owing to the need of guidance to the right 

 books with which to commence their studies. 



I trust that this good work will commend itself to you as 

 worthy of notice. ALEX. HiLL. 



Downing Lodge, September 17, 



Notoryctes typhlops. 



Allow me to protest against the misnomer " Marsupial 

 Mole " applied to Dr. Stirling's marvellous mammal by Mr. 

 Sclater, both in the Times and in Nature. " Mole-like Mar- 

 supial " it may be, but the other phrase has quite a different 

 meaning, and either shows a want of appreciation of important 

 characters, or implies a theory which, however plausible, has 

 not been proved. Alfred Newton. 



September 12. 



"W = M^." 



I WISH that Prof Greenhill would kindly explain to a bewil- 

 dered reader of your paper the nature of his quarrel with 

 " VV = M^," and with the writers of "theoretical" treatises 

 who use this equation. 



To those trained to regard quantity of matter as measured by 

 its inertia, and who regard the " mass" of a body as the quan- 

 tity of matter, so measured, which it contains, the equation 

 VV = M^ has a pretty clear meaning. 



A certain body "has a mass M," this being the measure of 

 its inertia in terms of that of the mass-unit. This body is 

 observed to have an acceleration g. We argue, from Newton's 

 experimental laws, that there is a force acting on it ; and we 

 measure this force by a number which is the product of the two 

 numbers, M (the measure of the mass of the body), and ^ (the 

 measure of the acceleration observed). 



NO. I 143, VOL. 44] 



If we observe a tight string attached to the body in question, 

 and have every reason to believe that there is no other cause for 

 the observed acceleration, we say that M^ measures the tension 

 T of the string; or write "T = M^." If the acceleration be 

 due to the presence of the earth only, we say that the earth 

 exerts a force [the "half" of the mutual stress] on the body, 

 measured by M^. This force we call the " weight of the body" ; 

 and the equation W = M^' gives us the measure of the "weight" 

 as deduced from the observation of rate of change of momentum 

 produced by it. 



If I felt sure that Prof. Greenhill considers M to be still 



W 

 merely a convenient abbreviation for — , I would say more on 



g 

 this matter ; but I am in doubt as to what are the views of 

 which he is so strong an opponent. 



I see that he wishes to abolish *'^" from works on hydro- 

 statics. Why ? I do not see how we can conveniently indicate 

 the dependence [cateris paribus\ of hydrostatic pressure on 

 the strength of the earth's gravitational field of force at any 

 given place otherwise than by the introduction of ^. But, as I 

 have already implied, I am as yet in the dark as to the precise 

 nature of the quarrel between Prof Greenhill and the theorists. 



Devonport, August 17. W. Larden. 



[We look to America for clear, unprejudiced ideas on the 

 definitions of elementary dynamics, and Mr. Frederick Slate's 

 letter from California is a valuable contribution, to which I hope 

 Mr. Larden has directed his attention. 



The quotations from certain elementary treatises which form 

 Mr. Larden's letter are the statements it was my chief object to 

 dispute ; according to this school of writers, the Standard Pound 

 Weight is not the lump of platinum preserved at the Exchequer, 

 but rather it is the pressure on the bottom of the box in which 

 it is kept. 



When goods are sold in commerce by weight, they are 

 weighed in scales, and the weight is the same wherever the 

 weighing is carried out, whether at the equator, or the poles, or 

 in the Moon, Sun, or Jupiter ; so that the weight cannot be said 

 to depend on the local value of g. the only effect of which is to 

 i-lightly alter the infinitesimal strain of the balance. 



Let Mr. Larden consult the recent Report of the Committee 

 on Electrical Standards, to see how carefully the units must be 

 defined to satisfy practical commercial requirements. — A. G. G.] 



When I was young, I never had the presumption to under- 

 stand the use of "^" in questions connecting mass and weight, 

 and I fear my boy takes after me. 



He told me the other day that he understood how a falling 

 body could have its velocity increased per second with a velocityof 

 g, or 32 feet per second ; and that he knew that m = stuff in a 

 body, and w = its weight, but he could not see what the 

 "blooming^" (I think that is what he called it) had to do with 

 the matter. 



I replied that no doubt, if we could only understand it, it had 

 a beneficent use in the economy of nature. 



Tommy Atkins, Senior. 



Sleep Movements in Plants. 



I READ the other day in a local paper that "Mr. Seemann, 

 the naturalist of Kellett's Arctic Expedition," states that plants 

 undergo sleep movements at regular intervals (presumably once 

 in 24 hours) during the long period when the sun never sets. 

 Has this been authenticated ? I thought it was well known that 

 a plant does not undergo periodic variations of the kind if it has 

 never been subjected to the regular succession of light and dark- 

 ness. Other instances are the daily periodicity of the strength 

 of so-called " root-pressure" and of the rate of growth. But if 

 the above observations are correct, not only have the sleep- 

 movements become independent of the ordinary determining 

 conditions in the individual, but they have become hereditary in 

 the species. If the movements really possess the significance 

 usually assigned to them (of checking excessive radiation) this 

 would seem to negative the prevalent view that the state of 

 panmixia alone suffices for the disappearance or degeneration of 

 a structure or mechanism. A. G. Tansley. 



September 19. 



