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ENCKE’S COMET, AND THE RESISTING 
MEDIUM. 
IT is well known to those interested in the subject, 
that one of the very few cases in which the celestial 
motions are not perfectly represented by the law of 
gravitation is afforded by Encke’s comet. Half a 
century has now elapsed since Encke announced that 
the comet, at each return, reached its perihelion two 
or three hours sooner than was expected. ‘This les- 
sening of the period was continually repeated at 
every return, thus showing the action of some con- 
tinuous cause. He proposed the hypothesis of a 
resisting medium, or atmosphere, surrounding the 
sun, as affording the most plausible explanation of 
the fact. Encke having died in 1865, the subject was 
next taken up at the Pulkowa observatory by von 
Asten. The latter died in 1878; and, after some delay, 
the work was continued by Dr. O. Backlund, a young 
Swedish astronomer of brilliant reputation. His 
second memoir on the subject has just been pub- 
lished, and includes a minute discussion of the obser- 
vations from 1871 to 1881, as well as a revision of 
some of Asten’s work. The most important result 
which he reaches is, that the acceleration is still going 
on, but is only half as great as that found by Encke. 
One of the anomalous results reached by von Asten 
was, that during the two revolutions from 1865 to 
1871 there was no acceleration. The revision of his 
work by Backlund shows, however, that this result 
was due to an error in some of the formulae which 
he used, and that, when this error is corrected, the 
effect is found to be continuous. 
No new light has been thrown on the cause of this 
result. Astronomers have not generally considered 
the acceleration as a well-established fact; because 
the accurate computation of the perturbations pro- 
duced by the planets, especially by Jupiter, is so in- 
tricate as to be very liable to small errors. The fact 
that Backlund has found an acceleration only half as 
great as that of Encke, shows that the method of the 
latter may be subject to doubt. At the same time, 
the amount of mathematical research which has been 
applied, and the constancy of the results found by 
all three investigators, now seem to leave little doubt 
of the fact. One reason for doubt has been that no 
other comet exhibited this retardation. This has 
been especially shown to be true of Faye’s comet. 
But the difference in the two results may be fully ex- 
plained by supposing that the atmosphere which re- 
sisted Encke’s comet does not extend far beyond the 
orbit of Mercury, the fact being that Faye’s comet 
does not even come within the earth’s distance from 
the sun. There is no other comet on which the 
hypothesis can be tested. 
The possibility that there is an exceedingly rare 
atmosphere around the sun is well worthy the atten- 
tion of astronomers and physicists. The zodiacal 
light, the motion of the perihelion of Mercury, and 
the acceleration of Encke’s comet, all point in this 
direction. The strongest evidence is afforded by the 
zodiacal light, because this shows that matter of. 
some sort exists within the region referred to. But 
SCIENCE. 
on 
[Vox. IIL, No. 69. 
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hitherto no means have been found to decide whether 
this matter is in a gaseous form, or in that of minute 
particles. In the latter case, the total mass would be 
too small to produce any effect upon the motion of 
either a planet or a comet. In the former case, how- 
ever, we cannot assign any such limit. Researches 
into this subject under favorable conditions are 
greatly to be desired, especially observations upon the 
zodiacal light at some elevated point near the equator. 
As science becomes more extended, it is to be hoped 
that stations for observations will be selected with 
less regard to local considerations, and with more 
regard to the conditions of scientific success. 
THOUAR AND CREVAUX. 
THE final report of Thouar, on his search for 
survivors of the Crevaux party, has been made pub- 
lic, and terminates the record of that gallant but 
unfortunately fruitless expedition to which reference 
has several times been made in these columns. By 
fruitless it is not intended to convey the idea that 
valuable results for geography and ethnology have 
not been attained by Thouar, but merely that the 
forlorn hope of rescuing alive any of Crevaux’s party 
was disappointed. 
After traversing with great haste the high plateau 
of Bolivia between La Paz and Oruro, experiencing 
a temperature of zero, Sucre, Tarija, and Caiza 
were successively reached. Conferences were had 
with all who seemed likely to afford information 
or advice; and letters written in French, Spanish, 
and the native dialect, were sent out among adjacent 
tribes. But even at this time there were no survivors. 
All that he could rescue was a broken barometer; a 
letter of Crevaux; a sketch-map of the Pilcomayo, 
prepared by Crevaux, and annotated by Billet; and a 
piece of one of the boats. On the 11th of September, 
1883, he reached the spot where the massacre took 
place, where a photograph was taken, and two wooden 
crosses erected in memory of the victims. 
The dead were cut in pieces by the Tobas, and 
each chief carried to his camp one of these ghastly 
trophies. The attack was prompted solely by a 
desire for vengeance. When Thouar inquired why 
they had killed Crevaux, who was so kindly disposed 
toward them, they invariably replied, ‘‘ We killed 
your brother because those of your color have killed 
ours.”? The dead were dismembered on the very 
spot where, a few days before, several Tobas had 
been shot by some inhabitants of Caiza. Thouar 
hardly finds it in his heart to blame the Tobas, who 
had been wantonly assailed, and who know no dis- 
tinction between white men. ‘‘ The Toba,’’ he says, 
“‘is strong, muscular, above the middle height. He 
has a dignified and impressive carriage. His color 
is that of old mahogany; his face is framed in long 
hair, black and straight ; his forehead is narrow ; 
the eyes slightly oblique; his cheeks prominent; his 
nose thick, broad, flattened at the tip; his mo 
large. He occupies himself exclusively in fishi 
and hunting. His face, breast, and arms are e 
