March 22, 1906] 



NA TURE 



" expansion " or " shear," is not the name of a distinct 

 kind of motion — it is only a term introduced to abbreviate 

 the discussion of a particular and important case of the 

 relative translation of the particles of a body. Direct 

 kinematical statements can only be made concerning 

 particles of infinitesimal volume; such particles can only- 

 have translation, they cannot rotate. When bodies of 

 finite volume are considered, they are analysed into particles 

 the motions of which are then investigated. If there is 

 no relative translation between the particles the motion 

 is said to be pure translation ; if there is relative transla- 

 tion the motion is said to be partly, or wholly, rotational. 

 It is the characteristic of rotation that two particles 

 situated on a straight line through the " axis of rotation " 

 possess a relative acceleration along that line, and it is 

 by the existence of these accelerations that absolute rota- 

 tion is detected. If we can find a line such that any two 

 particles situated on a line intersecting it are subject to 

 relative accelerations along the latter, the body is said to 

 rotate. It would be impossible for any observer on a rigid 

 body to detect its rotation, for the relative accelerations 

 of its particles could not be observed. If Foucault's 

 pendulum were rigidly attached to the earth, or if the 

 water in Newton's bucket were frozen, no observers on the 

 earth or the bucket having cognisance of these bodies only 

 could detect the " absolute rotation." In fact, the absolute 

 rotation of bodies of finite volume is only a special case of 

 the relative translation of particles. 



Norman R. Campbell. 

 Trinity College, Cambridge, March iS. 



Interpretation of Meteorological Records. 



The series of curves given by Messrs. Lander and Smith's 

 instruments, and published in Nature of March 15, are 

 most interesting, and one cannot help looking for the cause 

 of the close relation between the movements of all the five 

 instruments. It is with the view of offering an explan- 

 ation of the sympathy between these instruments that 

 the following lines are written. If I might venture to 

 suggest a first cause of these movements, I would say it 

 was the thunderstorm that drew the trigger which started 

 all of them. The thunderstorm gave rise to a heavy 

 fall of rain — a quarter of an inch in a few minutes — 

 and this rainfall appears to have been the cause of the 

 movement of all the instruments, and instead of being 

 placed last in the series should have been put first. The 

 effect of a heavy local fall of rain is to cause a down rush 

 of air, the air being dragged downwards by the falling 

 rain. This downward moving mass of air checks the 

 wind, because its movement is at right-angles to the wind, 

 hence the drop in the wind-velocity curve. The wind not 

 being able to pursue its course gets deflected — in this case 

 the curve shows it was to the north-west. The down rush 

 of air where it meets the surface of the earth has its 

 velocity reduced and direction of movement changed ; its 

 pressure is therefore increased, and the barograph shows 

 that the pressure increased by the tenth of an inch. The 

 downward rush of air would bring the air from the upper 

 strata to the surface of the earth, and as this upper air 

 would be in all probability the colder, it would cause a 

 fall in the temperature, which the thermograph shows 

 amounted to twelve degrees. 



On one occasion I had an opportunity of seeing this 

 downward movement of air produced by local rain. It 

 was while making some meteorological observations on 

 the top of the Eiffel Tower, in Paris. At first the weather 

 was fine, and the dust-counter showed that the impure 

 city air came to that height in great quantities. After a 

 lime a heavy shower came on which reduced the number 

 of particles in the air, and at last the air became as free 

 from dust as any air I have ever tested on the mountain 

 lops of Switzerland. This increase in purity could only be 

 due to the rain dragging down the upper purer air to the 

 level of the top of the tower, as rain cannot wash the air 

 to anything like that purity. 



If the time scale of the curves in the instruments had 

 been a good deal wider, and all the clocks going together, 

 one could have found out whether the above explanation 



no. 1899, vol. 73] 



was correct or not, as we would expect the rainfall curve to 

 begin first and all the others to follow, but from the close- 

 ness of the time scale of the curves no satisfactory in- 

 formation on this point can be obtained. 



Ardenlea, Falkirk, N.B. John Aitken. 



Agricultural Education and Colonial Development. 



In your issue of January n reference is made to the 

 requirement which has recently arisen for specialists in 

 agriculture and the allied sciences for employment in the 

 British _ colonies and dependencies. The case, so far as 

 India is concerned, may be stated very briefly. The 

 Government is willing to spend money in the development 

 of agricultural education and research, but the efficient 

 recruitment of the department — or, more properly, depart- 

 ments, for there are eight local governments in India and 

 Burma, each of which will have its own separate agri- 

 cultural department — is not an easy matter. The educated 

 native of India has not hitherto devoted the interest to 

 the study of agriculture that he has to law and medicine, 

 and men qualified to give instruction or conduct investi- 

 gations in relation to this national industry are not to be 

 found in the country. It is quite unnecessary to raise the 

 question as to whether they will be obtainable in the 

 future. This is one of the great desires of the Indian 

 Government. In the meantime, however, men qualified to 

 fill the offices above indicated are required, and a search 

 has to be made elsewhere. In this respect, then, India 

 appears to be drawing upon the same market as other 

 countries. 



In my view, the description of man that is required is 

 one possessing a thorough knowledge of principles. The 

 conditions of tropical agriculture are so very different from 

 those of the British Isles that it is highly desirable for the 

 Britisher to commence work in other continents with as 

 open a mind as possible. I am not thinking so much of 

 the agriculturist as of the botanist, entomologist, or 

 chemist. Just as the chemist who has made himself master 

 of pure chemistry makes eventually the best technical 

 chemist, and finds it, indeed, easier to apply himself to 

 any special technology than the so-called technical chemist, 

 so, likewise, for agriculture in foreign countries, the men 

 who will be most useful in the future will be those who 

 have obtained a thorough knowledge of their particular 

 science at college without any special reference to British 

 agriculture. J. Walter Leather. 



Agricultural Research Institute, Pusa, Bengal, 

 February 28. 



Peculiar Ice Formation. 



In reply to Mr. James Foulds's inquiry in Nature 

 (March 15, p. 464) whether the prismatic forms of ice such 

 as he has recently observed in Lancashire have been 

 observed elsewhere, it may suffice to refer him to previous 

 volumes of Nature, more particularly to vols. xxxi. and 

 xlvii., for letters from Messrs. Woodd-Smyth and McGee, 

 also from myself. In the latter volume is an account by 

 me of a more extended series of observations on these 

 " crystallites " than previous observers appear to have 

 made. Friability of soils is due to interstitial water. 



Bishop's Stortford, March 16. A. Irving. 



I observed the same formation as that described by Mr. 

 Foulds (p. 464) on bare soil, previously soaked with water, 

 near Champery, in Switzerland, as winter frosts began ; 

 and I believe that I have observed it everywhere as a 

 common phenomenon. 



I take it that the wet surface is first frozen, and that, as 

 the cold penetrates, the ice exudes from the soil much as 

 lanoline exudes from a lanoline tube, the water expanding 

 as it freezes, and so forcing its way out between the more 

 compact masses of soil, lifting the frozen surface-sheet. 



The first touch of sun caused the structure to break up. 



It struck me at the time that this was the cause of the 

 injurious effect of frost on the surface of roads considered 

 from the cyclist's point of view. W. Larden. 



Devonport, March 16. 



