August i8, 1910] 



NATURE 



:i9 



duction of the liorizontal motion of the earth at the 

 observing station. 



Primarily, the seismograph is a horizontal pendulum on 

 "he principle of Dr. Milne's apparatus, but instead of re- 

 ducing friction to a small amount. Prince Galitzin increases 

 it until the pendulum reaches the limit of " aperiodicity " 

 or " dead-beatness." Electromagnetic damping is so 

 strictly proportional to the velocity that it is immensely 

 superior to other forms of frictional damping. A copper 

 plate attached to the pendulum moves across the field 

 produced b\' two strong horse-shoe magnets, and the pole 

 distance is adjusted until the required degree of damping 

 is attained. There is no other friction, as the pendulum 

 is carried by steel wire. 



.\ heavily damped and truly dead-beat pendulum repro- 

 iuces with great precision the motion of the earth, which 

 jets the pendulum in motion. 



The actual movement of a heavily damped pendulum is 

 very small, but Prince Galitzin has shown how to magnify 

 the movement to any required degree without loss of 

 accuracy. A coil of fine wire attached to the pendulum 

 moves in the field of a second pair of horse-shoe magnets, 

 and the currents so generated in the coil when the 

 pendulum moves are measured by a D'.Arsonval galvano- 

 meter which has the same natural period as the pendulum, 

 but the circuit resistance is chosen so that the galvanometer 

 is also " dead beat." It can be shown that, except for a 

 few special cases, the motion of the galvanometer mirror 

 is a precise reproduction of the pendulum motion, and 

 hence of the earth motion. By means of an experimental 

 table on which the instruments are tested, Prince Galitzin 

 has shown that impressed motions of great complexity are 

 faithfully reproduced on the galvanometer. It is true that 

 the final scale of magnification differs for different periods 

 of incident waves, but in actual seismological practice it 

 is comparatively easy to allow for this. 



The registration is made photographically by reflecting 

 light from a fixed source and producing a dot of light on 

 a strip of photographic paper carried on a rotating circular 

 drum. The paper moves at a rate of 3 cm. per minute, 

 and the drum is carried sideways, so that it can be used 

 for a twelve-hour record. The trace is interrupted every 

 minute by an electrical time break, and so the times can 

 be estimated with ease to half a second. 



The pendulum and galvanometer are duplicated, so that 

 two records are obtained on the paper. One of these gives 

 the motion of the earth from north to south and the other 

 the motion east to west. They are arranged so that a 

 movement of the earth to north or to east is represented 

 by a motion of the corresponding dot up the sheet. 



Prince Galitzin has shown that the ratio of the ampli- 

 tudes of the very first impulse on the two pendulums gives 

 the tangent of the azimuth of the epicentre. He has proved 

 practically that this determination of the direction is 

 accurate to \° of arc. 



The usual method (Wiechert ZoUpritz) of calculating the 

 distance from the times of occurrence of the first and 

 second phases is followed. 



Thus a pair of Galitzin seismographs enable an observer 

 at a given station to determine the epicentre of an earth- 

 quake with great precision. Further, one can follow the 

 complete movement with confidence, although a certain 

 amount of computation is strictly necessary when waves 

 of widely different periods actually occur. Intelligent 

 examination of the record shows when this is necessary. 



Identity of natural period of pendulum and galvano- 

 meter and strict " aperiodicity " is the ideal. In practice 

 it is sufticient to get approximate equality, and determine 

 the differences by a simple process of standardisation. 

 For information on this point the reader is referred to 

 Prince Galitzin 's original memoirs in the publication of 

 the Imperial .'\cademy, St. Petersburg. G. W. W. 



RECEXT AGRICULTURAL PUBLICATIONS IN 



GREAT BRITAIN.^ 

 'T'HE Board of .Agriculture issues each month a Journal 

 intended for farmers,* small holders, and others in- 

 terested in agriculture, and, in order that it shall be 

 accessible to all, the price is fixed at the very low sum of 



1 Journ.il of the Bo.ird of Agriculture, and Supplerr.enls ; Journal of the 

 Royal Agricultural Society ; Agrxultural Students' Gazette. 



NO. 2129, VOL. 84] 



fourpence. It represents a very laudable, and on the whole 

 successful, attempt to bring to the farmer the best scientific 

 work on the problems confronting him ; it also deals with 

 matters of agricultural importance in foreign countries. 



A large proportion of the papers is devoted to plant 

 diseases. In a recent issue, Mr. Salmon describes the 

 Sclerotinia disease of the gooseberry. The mycelium of the 

 fungus penetrates into the tissue of the stetii, permeating 

 the cortex and the bast ; in time the stem is completely 

 " ringed " and the bush dies. Sometimes the attack is not 

 in the main stein, but in the young wood, the leaf, or the 

 berry. Burning all infected old and dead wood has been 

 found the best method of control. Mr. Salmon points out 

 that this fungus is capable of developing vigorously on dead 

 parts of the bush, while the American gooseberry mildew, 

 with which growers have sometimes confused it, is not. 

 Where burning is not practicable, recourse may be had ttt 

 spraying with copper sulphate solution. 



Other papers deal with possible improvements in agri- 

 cultural practice. Of these, one is by Mr. Priestley on 

 the eftect of overhead electrical discharges on plant growth.- 

 It has been known for some time that an electrical dis- 

 charge in some way increases plant growth, but the prac- 

 tical difficulties have only recently been overcome by the 

 use of Sir Oliver Lodge's high-tension valves, in conjunc- 

 tion with a Ruhmkorf coil of a type similar to that used in 

 X-ray work. The physiological problems are not yet 

 solved, and the practical application of the method is, of 

 course, very limited so far as can be seen at present. But 

 the fastidiousness of the present generation demands fruit 

 and vegetables in unnatural seasons, and the market gar- 

 dener, in furnishing the supply, is prepared to adopt any 

 methods likely to increase the rate of growth, provided the 

 cost is not too high. Mr. Priestley's data show that a good 

 case can be made out for the use of electrical discharges. 



Supplements to the Journal are periodically issued, each 

 dealing with a special subject. One deals with the work 

 of the International Agricultural Institute, but it is not 

 very clearly arranged. .Another gives in extenso the papers 

 read at the discussion on wheat at a joint meeting of 

 Sections B and K with Subsection K of the British 

 .Association last year. As this discussion has already been 

 dealt with in these columns, it is unnecessary to say any 

 more here ; the plan of printing and distributing widely 

 these British -Association discussions is admirable, and will, 

 we trust, be acted upon again. 



Under the able guidance of Mr. Mackenzie, the Journal 

 of the Royal Agricultural Society is steadily making up lost 

 ground. This Journal has had a remarkable history. The 

 early volumes, from 1840 (when it began) to about i860, 

 contain some of the finest papers that have been written 

 on agriculture and agricultural science. Then it had a 

 long series of lean years, due, no doubt, in part to the 

 decadence of agricultural science in England during that 

 period. It is now taking its place in the remarkable 

 revival that the last few years have witnessed. Mr. Hall 

 gives a connected account of the various investigations he 

 and other workers at Rothamsted have made on the 

 secondary effects of manures on the soil. Sulphate of 

 ammonia tends to make the soil acid, and therefore infer- 

 tile, unless sufficient lime is present ; the acidity is caused 

 by certain fungi in the soil which decompose the salt to 

 obtain the ammonia and thereby set free sulphuric acid. 

 On the other hand, nitrate of soda tends to make the soil 

 alkaline and to get it into a sticky, unworkable condition. 

 The decomposition is in this case brought about at the 

 roots of the plant in some way not yet understood, and 

 results in the acid radicle entering the plant while the 

 base remains outside as sodium carbonate. The latter 

 substance is known to deflocculate clay, and is the cause 

 of the unfavourable soil condition thus induced. Suitable 

 methods of treatment are suggested. 



Mr. F. H. -A. Mat-shall discusses some of the physiolo- 

 gical problems of the stock-breeder. This subject has been 

 curiously neglected, and it is of interest that Cambridge, 

 which has taken the lead in plant-breeding work on 

 Mendelian lines, should now be turning its attention to 

 stock-breeding. The practical man has acquired by long 

 experience a vast fund of information, which, however, 

 needs sorting out and systematising. Apart from the 

 problems involved in raising pedigree stock — a highly 



