274 
NATURE 
[ Yan. 19, 1882 
the medium, its own mass being greater, its velocity will 
be less ; its motions will therefore, relatively to the pul- 
sator that is acting on it be in the same phase, and it will 
be attracted. On the other hand a body lighter than 
the surrounding water vibrates with a greater amplitude ; 
and being, relatively to the pulsator, in an opposite phase, 
will be repelled. 
Lastly, we give a drawing of the ingenious in- 
dicator by which Prof. Bjerknes has succeeded in 
tracing the lines of force in the midst of the medium 
through which such actions as these are propagated. For 
this purpose a light hollow metal egg-shaped vessel held 
by a thin flexible steel wire to a heavy foot is placed in 
the tank. It takes up, both in magnitude and direction, 
any oscillating movement which may be going on at that 
part of the liquid in the tank where it is placed. If, as in 
Fig. 6, it is desired, for example, to investigate the lines 
of hydrodynamic force in the ‘‘field” between two pul- 
sating tambours, we may explore these lines by placing 
the indicator at different positions and observing the 
direction in which it oscillates. A camel's hair pencil 
charged with colour fixed to the summit of the indicator 
serves to inscribe a trace of the line of oscillation upon a 
sheet of glass placed above. It is singular to observe 
with what fidelity the lines of force of various magnetic 
fields are reproduced in the figures obtained in the analo- 
gous hydrodynamic conditions. An oscillating sphere 
shows two poles ; a pulsating sphere radial lines only. 
Even the phenomena of the ‘‘field’’ of force due to 
electric currents can be imitated by Prof. Bjerknes’ appa- 
ratus. A cylinder rotating round its own axis with an 
alternately directed rotation represents an electric current. 
Near such an excitor an indicator in the liquid vibrates 
tangentially to the rotation. The remarkable magnetic 
figures produced by the mutual action of two currents 
upon one another are perfectly reproduced by the mutual 
actions (on an indicator) of two rotating cylinders ; while 
the spiral systems of lines of force produced, as Prof. 
Silvanus Thompson discovered (see NATURE, vol. xix. 
p. 83), by the mutual action of a magnet pole and of a 
current traversing it are exactly reproduced by Prof. 
Bjerknes’ indicator under the influence of an apparatus 
which pulsates and rotates synchronously. The most 
extraordinary thing about Prof. Bjerknes’ researches is 
that they are all the result not of haphazard experiment, 
but of careful and abstruse calculation. In 1865 he began 
the investigation. By 1875 he had perceived that the 
calculated motions were such as would have direct 
analogies with the phenomena of permanent magnets. 
Toward 1879 he found that these analogies might be 
extended to the case of magnetic induction. Then, and 
not till then, were the beautiful pieces of apparatus made, 
by means of which these remarkable previsions have | 
been verified. 
JOHN WILLIAM DRAPER 
OHN WILLIAM DRAPER, M.D., LL.D., President 
of the Medical and Scientific Faculties of the Uni- 
versity of New York, who died January 4, 1882, was an 
Englishman, having been born at St. Helens, near Liver- 
pool, on May 5, 1811. He was therefore in his seventy- 
first year. Up to the age of twenty-two he was resident 
in his native country, receiving his education, first under 
private tutors, and afterwards studied chemistry in the 
University Coliege, London, then knownas the University 
of London. In 1832 he emigrated to the States, and con 
tinued his studies at the University of Pennsylvania, 
where, in 1836, he took the degree of M.D. Meantime 
his talent for original research had manifested itself in 
the production of several memoirs, which appeared in the 
pages of the Journal of the Franklin Institution. The 
first of these (published in 1834) was ‘“‘ On the Nature of 
Capillary Attraction’’; whilst a second was devoted toa 
discussion of the most eligible method of constructin 
galvanic batteries of four elements. In 1835 he published 
an account of some experiments made to detect whether 
light exhibits any magnetic actions. Several branches of 
the science of electricity subsequently claimed his atten- 
tion. In 1839 he wrote a memoir, which afterwards was 
reprinted in the Philosophical Magazine, “ On the Use of 
a Secondary Wire as a Measure of the Relative Tension 
of Electric Currents.’’ It is instructive to observe in this 
memoir how Draper’s exact mind revolted against the 
misuse, by writers on electricity, of the words “tension”’ 
and “intensity’”’; and, though he himself employed both 
terms, he carefully distinguished between them, using 
“tension” for what we now call ‘‘ electromotive force,” 
and “intensity” for the “strength of the current,” agree- 
ing therefore with the practice of-many continental 
authorities. He also made experiments upon electro- 
capillary motions, and contributed to the science of 
thermo-electricity, a valuable series of determinations of 
the thermo-electromotive force of different pairs of metals 
at different temperatures. In 1837 began the notable 
series of researches upon the nature of rays of light in the 
spectrum, with which the name of Draper will always be 
associated. His paper that year bore the title *‘ Experi- 
ments on Solar Light,’ but it failed to attract much 
attention in Europe. He was now devoting himself to 
photography and photo-chemistry with great zeal. His 
paper “ On the Discovery of Latent Light,” in 1842, dealt 
with the images produced by rays of light which are only 
subsequently developed by some chemical reaction—a 
process with which the art of photography has made us 
familiar, but which was then a curious and novel pheno- 
menon. It was Draper who first discovered that in the 
ultra-violet part of the spectrum there are absorption 
bands like the Fraunhofer lines in the visible part of the 
spectrum. To enumerate the works which proceeded 
from Draper’s pen upon the chemical and physical pro- 
perties of the ultra-violet, or as he styled them, “¢hontc 
rays, would be inadmissible here. Suffice it to say that 
the greater part of the fifty memoirs mentioned in the 
Royal Society’s Catalogue related to this subject, and the 
most important of them are to be found reprinted in his 
“‘ Scientific Memoirs,” published in 1878. In this volume 
may be feund the pregnant suggestion for a standard of 
white light for photometry of a piece of platinum foil of 
given size and thickness, raised to a white heat by an 
electric current of specified strength. To guard against 
fusion he suggested that an automatic short-circuiting 
apparatus should be constructed by some ‘skilled arti- 
ficer.” He thus exactly anticipated Edison’s first incan- 
descent lamps: though the satisfactory standard of white 
light appears to be as far off as ever. 
The latest papers Draper published were entitled “ Re- 
searches in Actino-chemistry,’’ and treated of the distri- 
bution of heat and of chemical force in the spectrum. 
They appeared in 1872 in the American Journal of 
Science and in the Philosophical Magazine. During 
these years of work Draper held important appointments, 
first in Hampden-Sidney College, Virginia, where he was 
Professor of Chemistry, Natural Philosophy, and Physio- 
logy, and afterwards (1839) in the University of New 
York, where he was Professor of Chemistry and Natural 
History, a post modified two years later into that of Pro- 
fessor of Chemistry in the Medical College of the Uni- 
versity. In addition to the original memoirs enumerated 
above Dr. Draper wrote several valued text-books ot 
science ; a Text-book of Chemistry in 1846, and a Human 
Physiology in 1856, both of which works went through 
several editions. 
Dr. Draper's literary activity manifested itself however 
in other directions, and he has left an enduring mark in 
literature as a philosophic historian of no mean merit. 
The “ History of the Intellectual Development of Europe,” 
| published in 1862, has been translated into all the current 
