SCIENTIFIC SUMMARY. 
361 
giving' a brief account of a number of memoirs and papers which have 
appeared to me to possess more than a passing interest.’ Mr. Dreyer’s sum- 
mary will certainly be found of great use to all astronomers ; and it is to be 
trusted that in the interest of Astronomy its compiler will see his way to 
rendering it an annual publication. There are few things which a scientific 
man requires more than a compact record of the principal work achieved 
during the year — a record giving not only a general idea of the contents 
of a paper or a memoir — the mere title is of far less value — but the publica- 
tion wherein it appeared. Mr. Dreyer divides his record into the following 
seventeen heads: — 1. Spherical Astronomy; 2. Theory of Instruments; 3. 
Celestial Mechanics; 4. The Sun; 5. The Moon; 6. The Intra-mercurial 
Planet Question ; 7. Planets and Satellites ; 8. Comets ; 9. Meteors and 
Meteorites ; 10. Fixed Stars ; 11. Annual Parallax : 12. Double Stars ; 
13. Nebulae and Clusters ; 14. Photometry ; 15. History of Astronomy, 
Bibliography ; 16. Observatories ; 17. Miscellaneous Notes. 
Figure of Mars. — During the last opposition of Mars in November 
1879, Professor C. A. Young made a numerous series of measures of the 
diameter of the planet with a parallel wire micrometer, on the 9 1 -inch Alvan- 
Clark Equatorial of the Princetown College Observatory. From a careful 
discussion of these observations, Prof. Young deduces the following value for 
the apparent equatorial and polar diameter of the planet on November 12, 
1879 : — 
Equatorial diameter of Mars . = 20'634 ± 0*034 
Polar diameter of Mars . . = 20*552 ± 0*043 
Mean diameter of Mars . . = 20*593 ± 0*035 
These measures, being made with a parallel-wire micrometer, must be affected 
by the constant error due to irradiation at the edge of the planet, the 
irradiation being principally due to the spurious disc into which every 
point of light is converted by the telescope. Prof. Young points out 
that if we adopt as the true value of the diameter of the planet 
Mars at the distance unity from the Earth, the value 9"*352 deduced 
by Hartwig from all the measures prior to 1879, the real value of the 
diameter of Mars on November 12, 1879, must have been 19"*128. The 
difference l // *465, between this and the value found by Prof. Young, which 
was 20 // *593, is regarded by him as being due to irradiation. This is larger 
than that due merely to the spurious telescopic diffraction disc, which would 
amount to about 1"*0. Prof. Young points out that the value found by him 
for the mean diameter of Mars, when reduced to distance unity, is 10"*068. 
Prof. Young then utilizes these measures to determine the figure of the 
planet. The difference between the apparent polar and equatorial diameter 
is 0"*0818, which would correspond to a compression or ellipticity of 1 -f234. 
But as Prof. Young points out, at the time of measurement the pole of Mars 
was not really on the periphery of the planet, but was 14°*5 from it, so that 
the apparent polar diameter was greater than the real polar diameter. 
Allowing for this, Prof. Young deduces 1 -f 219 as the actual polar compres- 
sion of the planet Mars. As Prof. Young points out, this is in close accord 
with the value 1 - 228 deduced by Prof. J. C. Adams for the polar com- 
