788 
sumption of only a moderate polar com- 
pression of the planet. The orbits based 
upon the measures of Professors Newcomb, 
Hall, Holden, Hermann Struve and A. 
Hall, Jr., were best satisfied by a uniform, 
progressive change of the above elements. 
From a series of observations by Barnard 
in 1897-98, and another by himself, the 
author has been enabled to show that there 
is a variation in the rate of this change. 
The result of a least square solution from 
all the available elements from 1848 to 
1898 was given in a table. Although the 
residuals are but slightly smaller than 
would result from the assumption of a uni- 
form rate, this is chiefly due to the inac- 
curacy of the earlier elements. The va- 
lidity of a variable rate is further confirmed 
by an apparent increase in the mean motion 
of the satellite referred to the movable node 
of the orbit plane on the Earth’s equator, 
which is otherwise difficult to explain. 
The equatorial protuberance of the planet 
will cause the pole of the satellite’s orbit to 
describe, with uniform retrograde motion, 
a small circle around the pole of the planet’s 
equator considered as a fixed point, while 
the inclination of the two planes will re- 
main constant. The node of the satellite’s 
orbit on the planet’s equator will thus move 
uniformly along the latter, in a direction 
contrary to the motion of the satellite. 
The differential equations of this motion 
were given. 
A least square solution of 36 equations of 
condition gave the following elements of the 
planet’s equator: 
Y 17°.84 
Io 49°. 38 
6 119°.16 } Equator of 1850.0 
No 22°.02 | 
AG -0°.677 
Aare 1 
The flattening is « — 102.9 
The period of the revolution of the pole 
of the satellite’s orbit is 531.75 years. The 
SCIENCE. 
[N.S. Vou. X. No. 257. 
flattening is a quantity represented by a 
difference of only 0.03 between the polar 
and equatorial diameters of the planet—a 
quantity scarcely measurable in the most 
powerful telescope. 
(To be published in the Astronomical 
Journal. ) 
A.8. Frint: The Repsold Transit Micrometer 
of the Washburn Observatory and Slat 
Screen Apparatus. 
The transit micrometer is at present em- 
ployed on the meridian circle, of 12.2 cm. 
aperture, in a new series of observations for 
stellar parallaxes by the method of meridian 
transits. The essential feature of the in- 
strument is an auxiliary screw which is 
geared with the regular right ascension 
screw and carries the eye-piece to follow the 
movable thread. The observer keeps a star 
image bisected continuously by means of 
two turning heads on this auxiliary screw, 
one on each side of the micrometer box, 
while a series of contact points on the head 
of the right ascension screw effect the elec- 
tric signals on the chronograph. The present. 
observer finds the probable error of a single 
signal to be 0.030s., the same as in the ordi- 
nary method with fixed threads. 
This micrometer has also a device for ob- 
taining a record of several bisections on a. 
given star in declination without the neces- 
sity of reading the micrometer head. A 
lever clamped to the declination screw 
travels over a short graduated are and, 
being pressed down by the observer, pricks 
a mark on a strip of paper. 
Some suggestions were made for minor 
improvements, but the performance of the 
micrometer as a whole has been found very 
satisfactory. 
The slat screen apparatus, designed by 
Professor Comstock, is intended to furnish 
the simplest means of diminishing the ap- 
parent magnitude of brighter stars. A frame 
