92 



NA JURE 



[May 24, 1 S8j 



length (o ily used lo measure the ratio of the distance between 

 the candle- to the distance of the grating from either). The 

 experiment showed the distance from centre to centre of con- 

 secutive liar, of the grating to be 32 times the wave-length of 

 yellow light. This being remembered to be 5S9 x lo- 5 of a 

 centimetre, it was concluded that the breadth of the space on 

 which the 250 hues are engraved is 250. 32. 5'892. io- 5 , or 

 •4714 of a centimetre ! According to the instrument-maker it is 

 really -5 of a centimetre ! Five minute- spent on the experiment 

 instead of one, and sodium flames beiind fine slits, instead of 

 open candles blowing about in the air might easily have given 

 the result within one-half per cent, instead of 4^ per cent. 

 Thus the cosmic traveller can easily rec iver his centimetre and 

 metre measure. To recover his unit of time is less easy. One 

 way is to go through Foucault's experimental determination of 

 the velocity of liy,ht. 



But he must be imagined as electrically-minded ; and he will 

 certainly, therefore, think of "»," the number of electrostatic 

 units in the electro-magnetic unit of electricity ; but he will, 

 probably, see his way better to doing what he wants by making 

 for himself a Siemens' mercury unit (which he can do easily, 

 now that he has his centimetre), and finding (by the British 

 Association method, or Lorenz's with Lord Rayleigh's modifi- 

 cation, or both), the velocity which measures iis re istance in 

 absolute measure. This velocity, as is known from Lord 

 Raylei^h and Mrs. Sidgwick, is 9413 kilometres per mean solar 

 second, and thus he finds, in mean solar seconds, the period of 

 the vibrator, or arbitr.iry-unit chronometer, which he used in his 

 experiments. 



Still, even though this method might be chosen as the 

 readiest and most accurate, according to present knowledge, of 

 the fundamental dati for recovering the mean solar second, the 

 method by " v " is to 1 interesting and too instructive in respec- 

 to elimination of properties of matter from our ultimate metrical 

 foundations to be unconsidered. One very simple way of ex- 

 perimentally determining "v" is derivable from an important 

 suggestion of Clark and Bright's paper, referred to above. Take 

 a Leyden jar, or other condenser of moderate capacity (for 

 example, in electrostatic measure, about 1000 centimetres), 

 which must be accurately measured. Arrange a mechanism to 

 charge it to an accurately measured potential of moderate 

 amount (for example, in electrostatic measure, about 10 c.g.s., 

 which is about 3000 volt-), and discharge it through a galvano- 

 meter coil at frequ-mt regular intervals (for example, ten times 

 per second). This will give an intermittent current of known 

 average stre .gth (in the example, lo 5 electrostatic c.g.s., or 

 about 1/300,000 c.g.s. electromagnetic, or 1/30,000 of an am- 

 pere), which is to be measured in electromagnetic measure by an 

 ordinary galvanometer. The number found by dividing the 

 electrostatic reckoning of the current, by the experimentally 

 found electromagnetic reckoning of the same, is "v," in centi- 

 metres per the arbitrary unit of time, which the experimenter 

 in search of the mean solar second has used in his electrostatic 

 and electromagnetic details. The unit of mass wbicli he has 

 chosen, also arbitrarily, disappears from the resulting ratio It 

 is to be hoped tint before long " v " w \\] be known within 1/10 

 per cent. At present it is only known that it does not probably 

 differ 3 per cent, from 29 X io 10 centimetres per mean solar 

 second. When it is known with satisfactory accuracy, an 

 experimenter, provided with a centimetre measure, may, any 

 where in the universe, rate his experimental chronometer to 

 mean solar seconds by the mere electrostatic and electromag- 

 netic operations described above, without any reference to the 

 sun or other natural chronometer. 



The remainder of the lecture was occupied with an explana- 

 tion of the application of the absolute system in all branches of 

 electric measurement, and the definition of the now well known 

 practical units f mnded on it, called ohms, volts, farads, micro- 

 farads, amperes, coulombs, watts. The name mho, found by 

 saying ohm to a phonograph and then turning the drum back- 

 wards, was suggested for a unit of conductivity, the reciprocal 

 of re istance. The subdivision, millimho, will be exceedingly 

 convenient for the designation of incandescent lamps. 



The British Association unit has been found by Lord Ray- 

 leigh and Mrs. Sidgwick to be -9868 of the true ohm (io 9 centi- 

 metres per second), which differs by only 1/50 per cent, from 

 •9S70, the number derived fro 11 Joule's electrothermal measure- 

 ments described in the British Association Committee's Report 

 of 1S67, with 772 Manchester foot-pounds taken as the dynami- 

 cal equivalent of the thermal unit from the measurement 



described in bis Royal Society paper of 1849, and confirmed by 

 his fresh measurement of 20 years later, published in his last 

 Royal Society paper on the subject. 



It is satisfactory that, whether for interpreting old results, or 

 for making resis'ance-coils anew, electricians may now safely 

 use the British Association unit as "95568, or the Siemens u .it as 

 •9413, of the ohm defined as io 9 centimetres per second. 



U.S. NATIONAL ACADEMY OF SCIENCES 1 



""THE annual meeting of this body was held in Washington 

 ■*- during the last week, with an attendance of forty members. 

 Scientific sessions were held on Tuesday, Wednesday, and Fri- 

 day, in the large lec*ure-room of the National Mu eum, and 

 business sessions'on every day of the meeting. 



Twenty-four foreign associates were elected as follows : — 

 Astronomers : Prof. Otto von Struve, of the Imperial Observa- 

 tory at Pulkowa, Russia ; Prof. J. C. Adams, of Cambridge, 

 Eng. ; Prof. A. Auwers, Director of the Observatory at Berlin; 

 and Prof. Theo. von Oppolzer, Director of the Ob-ervatory at 

 Vienna. Mathematicians : Prof. Arthur Cayley, of the Uni- 

 versity of Cambridge, Eng. ; Prof. J. J. Sylvester, of the Johns 

 Hopkins University, Baltimore ; and Prof. E. Bertrand, of 

 Paris. Physicists : Prof. R. Clausius, of the University of 

 Bonn ; Baron H. von Helmholtz, Professor in the University of 

 Berlin ; Prof. Robert Kirchhoff, of the Univer-ity of Berlin ; 

 Prof. G. G. Stokes, of the University of Cambridge, Eng. ; and 

 Sir William Thomson, Professor in the University of Glasgow. 

 Chemists : Prof. J. B. Dumas, Secretary of the Academy of 

 Sciences, Paris ; and Professors M. Berthelot, Boussingault, 

 Chevreul, and Wiirtz, all of Paris. Geologist : Freiherr von 

 Richthofen, Professor in the University of Bmn, and President 

 of the German Geographical Society. Botanists : Sir J. D. 

 Hooker, Director of the Botanical Gardens at Kew, Eng. ; 

 Prof. A. de Candolle, of Geneva. Biologists : L. Pasteur, of 

 Paris ; Prof. T. H. Huxley, of London ; Prof. R. von Virchow, 

 of the University of Berlin ; A. von Kolliker, Processor of 

 Anatomy in the University of Wurzburg. Prof. Struve, one of 

 the newly elected foreign associates, who is on a visit to this 

 country, was a regular attendant at the scientific sessions of the 

 Academy, and read a paper. 



In c >n-equence of the death of Prof. W. B. Rogers, the 

 President, it became necessary to elect his successor. On the first 

 ballot, Prof. Wolcott Gibbs, f Cambridge, one of the founders 

 of the Academy, was elected. He, h nvever, firmly declined 

 the honour, from a feeling, as he said, that he c mid not give the 

 time nece-sary to the work. The Academy reluctantly acquiesced 

 in the decision of Prof. Gibbs, and proceeded to a second ballot, 

 when Prof. O. C. Marsh, of New Haven, the acting President, 

 was elected by a handsome majority. The newly-elected 

 President will hold office for six years. 



The first act of the new Preside t was to announce that he 

 had received from Mrs. Mary A. Draper, widow of Prof. Henry 

 Draper, the sum of six thou-and dollars, accompanied by a deed 

 of trust which fully specified the objects she had in view. He 

 called upon Prof. Rarker to explain the nature of the trust to 

 the Academy. Prof. Barker first made some appropriate re- 

 marks, recalling Prof. Draper's interest in the Academy, and 

 then read the deed, the sub-ta-ce of which is as follows :— The 

 income of the trust is to be used " for the purpose of striking a 

 gold medal which shall be called the ' Henry Draper Medal,' 

 shall be of the value of two hundred dollars," and shall be 

 awarded from time to time, but not oftener than o:.ce in two 

 years, as a premium to any person in the United States or else- 

 where who shall make an original investigation in astronomical 

 physics, the re-ults of which shall be deemed by the Academy 

 of sufficient importance and benefit to science to merit such 

 recognitio 1. If at any time the income of the fund shall exceed 

 the amount necessary for the striking of the medal, the surplus 

 may be used in aid of investi >a< ions and work in astronomical 

 physics to be made and carried on by a citizen of the Ui.ited 

 Sntes. 



The President appointed Messrs. G. F. Barker, W. Gibbs, 

 S. Newcomb, A. W. Wright, and C. A. Young as a committee 

 to have charge of the fund, to make rules to govern the award 

 of the medal, and to suggest to the Academy for approval the 

 names of those who may be considered worthy of the award. 



The Treasurer announced that in accordance with the will of 

 ' Froai Sctt/tce, April 27. 



