360 
POPULAR SCIENCE REVIEW. 
Mr. Neison is engaged in determining the solar parallax from a new class of 
inequalities in the motion of the Moon ; Messrs. Neison and Campbell, by 
means of a new determination of the parallactic inequality, by a method 
which is independent of the diameter of the Moon ; and Prof. Winnecke, by 
a method founded on observations of the distance of the planet Venus from 
small stars, when near its greatest elongation. 
Jupiter . — The planet Jupiter is again coming into a convenient position 
for observation, and it is probable that during the coming opposition further 
important contributions to our knowledge of its physical condition will be 
obtained. The great red spot still remains visible and apparently unchanged, 
and it crosses the centre of the disk of the planet within a few minutes of the 
time predicted by Mr. Marth, so that its period of rotation remains un- 
changed, and is not slowly altering, as was surmised by several observers 
early last year. White equatorial spots are again visible, having a swifter 
rotation than the red spot. The dark spots do not seem to be nearly so in- 
tense as during the early part of the year. 
The Comets . — This year has been a peculiarly fortunate year as far as 
comets have been concerned, no less than four having been already observed, 
and two of these comets of considerable brightness. 
Comet b, 1881. — This bright comet was discovered in the Southern Hemi- 
sphere, and was at first believed to be a return of the great comet of 1807, a 
view quickly found to be incorrect. In June it made its appearance above 
the northern horizon, and for some time remained a conspicuous object, 
having a bright nucleus and a tail of considerable length. Numerous 
sketches were made of its appearance and the usual evidences of apparent 
changes were seen, though, from the immense extent of these changes, it is at 
times difficult to believe that they can be other than optical. From, his 
observations with the polariscope, Prof. A. Wright, of Washington, is of the 
opinion that the greater amount of the light reaching us from the tail of the 
comet is merely reflected solar light. Prof. Young, of Princetown, considers, 
from spectroscopic observation, that we thus have almost overpowering 
evidence that the gaseous substance of this comet is a hydrocarbon, and in a 
molecular state corresponding to that at the base of a gas-flame. It is not 
necessarily at a very high temperature, since the experiments of Wiederman, 
Hasselberg, and others, tend to show that under certain circumstances gases 
may become luminous, and give their characteristic band-spectrum at 
temperatures even below that of boiling water. Indeed, the dynamical 
theory of gases would show that all gases at any temperature must be, to a 
certain extent, luminous, and give a spectrum if seen in sufficiently thick 
strata, whatever may be their temperature, an increase of temperature 
merely increasing the amount of light coming from any mass of gas. 
Prof. Henry Draper, of New York, obtained a number of photographs of 
the comet, — one with an exposure of 2 h 42 m , showing a very dense nucleus 
and a tail about 10° long : on this photograph a number of very small stars 
are to be seen, some shining through the central portion of the tail of the 
comet. Mr. A. A. Common, of Ealing, also succeeded in obtaining photo- 
graphs of this comet with his great 37-inch reflector, but with less success, 
having been unable to employ so long an exposure. 
Dr. Huggins has obtained photographs of the spectrum of the comet, and 
