i6 



NA TURE 



[May 2, 1 901 



The various phases of the oscillation did not take place 

 at the different stations at the same absolute time, or 

 local time, but in every instance were associated with the 

 time of maximum obscuration of the sun. The duration 

 of the oscillation was apparently about the same as that 

 of the eclipse, about two and a half hours. The range of 

 the oscillation was about one minute in arc for declina- 

 tion, and about eight units in the fifth decimal CCS. for 

 horizontal intensity, that is, to about 1 2800th part of the 

 absolute horizontal intensity. The general effect was to 

 deflect the declination needle to the west, and decrease 

 the horizontal intensity, before the time of ma.ximum 

 obscuration, the movement afterwards being in both 

 cases in the opposite direction. The analysis indicates 

 that the cause producing the magnetic oscillation was 

 situated outside of the earth's crust, the presumption 

 being very strong that the oscillation is to be referred to 

 some change produced in the upper atmospheric regions 

 by the abstraction of the sun's rays, due to interposition 

 of the moon. 



Dr. Bauer e.xpresses himself as having been in doubt 

 before making the observations as to whether any mag- 

 netic effect referable to the eclipse would reveal itself, 

 and adds that he was afterwards slow to conclude that 

 the magnetic oscillation observed was not accidentally 

 connected with the eclipse, until he had made such 

 exhaustive examination of every point involved as justi- 

 fied him in formulating a definite conclusion. The result 

 is interesting, and makes it desirable, as he says, that 

 every opportunity should in future be taken to obtain, 

 during eclipses, simultaneous magnetic, atmospheric- 

 electric and meteorological observations at as many 

 stations as possible. 



It is to be remarked that, although Dr. Bauer even- 

 tually speaks with some confidence as to the magnetic 

 movement observed having relation with the eclipse, the 

 movement in question was small, and, abstractedly 

 speaking, much too small on which to found any certain 

 conclusion, considering the abundance of magnetic move- 

 ments of similar and even greater magnitude. The 

 circumstance that seems really to give weight to the 

 conclusion drawn is the statement that the various phases 

 of the magnetic oscillation were associated with the time 

 of maximum obscuration of the sun. Confirmation of 

 this circumstance is therefore what is now to be desired. 



Following the paper there is printed an appeal for 

 international co-operation in magnetic and allied obser- 

 vations during the total solar eclipse of May 17 next. 

 William Ellis. 



PROF. H. A. ROWLAND. 



TTENRY AUGUSTUS ROWLAND was born in 

 ■*■ -•- 1848. He was educated as an engineer, and 

 graduated at the Rensselaer Polytechnic at Troy, New 

 York, in 1S70. After one year's experience as a railway 

 engineer on the Western New- York line, and a second 

 spent as instructor in natural science at Wooster, Ohio, 

 he returned to his college to share in its teaching, be- 

 coming an assistant professor in 1874. Two years later, 

 in 1876, after spending a year under Helmholtz in Berlin 

 he took office as the first professor of physics at the 

 newly founded Johns Hopkins University. Baltimore re- 

 mained his home until his death, on April 16, at the early 

 age of fifty-three years. 



His work at Berlin on the magnetic efforts due to a 

 rnoving body when carrying an electric charge brought 

 him at once into fame. The result was published by von 

 Helmholtz in 1876, and is thus described by Maxwell in 

 a metrical letter to Tait, written in June, 1877. Tait had 

 inquired, also in verse, as to the electric effects to be ex- 

 pected if a disc of ebonite carrying a charge were made 

 to rotate in its own plane, and Maxwell writes : 



NO. 1644, VOL. 64] 



The mounted disk of ebonite 



Has whirled before nor whirled in vain, 



Rowland of Troy, that doughty knight. 

 Convection currents did obtain, 



In such a disk, of power to wheedle 



From its loved north the subtle needle. 



Rowland showed by the direct effects produced on a 

 magnetic needle that a charged body in motion gave rise 

 to a magnetic field just as though it were a current whose 

 strength depended on the product of the charge and the 

 velocity. 



This result is of fundamental importance to electrical 

 theory ; it was confirmed by Rowland and Hutchinson 

 in 1889, and has been generally accepted as an estab- 

 lished fact. Within the last few months, however, 

 Cremieu has published an account of a repetition of Row- 

 land's experiments which has led him to a negative result ; 

 the question just at the present moment appears to need 

 further investigation. 



Rowland's appointment at Baltimore was rapidly fol.- 

 lowed by a series of brilliant researches, each of the first 

 importance. His determination of the unit of resistance 

 came first. This was published in 1878. The original 

 B.A. units were constructed by the Electrical Standards 

 Committee in 1863-4 to represent 10" C.G.S. units of re- 

 sistance ; according to Kohlrausch's results in 1870 they 

 were 2 per cent, too high, while according to Lorenz 

 (1873) they were 2 per cent, too low. Rowland's paper 

 contains an able criticism of the old experiments and a 

 detailed account of his own which led him to the number 

 •9912 X 10" C.G.S. units as the value of the B.A. units. 

 Further experiments in 1887 reduced this to -9864 x 10''. 

 The value now generally accepted is '98653 x 10'-'. Row- 

 land himself employed a modification of Kirchhoff's 

 original method, in which the induction current in a 

 secondary circuit produced by reversing a measured 

 primary current in a neighbouring circuit is observed. 



In 1879 Roxyland presented to the .•American Academy 

 of Arts and Sciences his paper on the mechanical equiva- 

 lent of heat, with subsidiary experiments on the variation 

 of the mercurial from the air thermometer, and on the 

 variation of the specific heat of water. To attempt to- 

 give any account of the contents of this classic work 

 would occupy too much space. To appreciate its value 

 and to realise the skill and the ingenuity of its author it 

 must be studied itself .More is known now about exact 

 thermometry and the precautions necessary in using a 

 mercury thermometer, and so it has come about that some 

 corrections are necessary in Rowland's work, specially in 

 that part of it which deals with the relation between the 

 scales of the mercury and the air thermometer. These 

 corrections were made at the Johns Hopkins University 

 by Messrs. Day and Wardner and Mallory ; but this fact 

 detracts nothing from the importance of his investigation, 

 and among the many determinations of the value of Joule's 

 equivalent, Rowland's will always remain in the first rank. 



Passing over, for the present, much work of great value, 

 among which we may note his investigations into the mag- 

 netic permeability of various substances, published in the 

 Philosophical Mai^azinc for 1873 and 1874, and his theory 

 of Hall's effect, we come ne.xt to the year 1882, when Row- 

 land gave to the Physical Society of London an account 

 of his concave grating. This is published m Xht. PhilO' 

 sophical Magazine for September, i SS3. 



The results of this discovery are well known. A new 

 weapon was placed in the hands of spectroscopists ; it 

 became possible to photograph spectra directly without 

 the use of prisms or lenses, and with a greatly increased" 

 dispersion and resolving power ; the beautiful maps issued 

 at a later date by Rowland himself, and by Higgs of 

 Liverpool, are striking evidences of the value of the 

 grating ; the additions to our knowledge arising from this 

 one discovery are already enormous ; much has been 

 achieved which, without it, would have been impossible. 



