O44 
Restorations of the fore and hind legs of Allosaurus 
are given. 
ity in size. A new classification of the order Thero- 
poda is also proposed, including the European as well 
as the American forms. 
THE ASTRONOMICAL LABORS OF MR. 
COMMON. 
In his address before the Royal astronomical soci- 
ety in February last, on the presentation of the gold 
medal to Mr. Common for his photographs of celes- 
tial bodies, the president of the society, Mr. Stone, 
remarked that the council, in making the award, had 
been less influenced by originality in the methods 
adopted than by the great practical success which 
has attended his efforts in this field of astronomical 
research. It will be of interest to note a few points, 
relating to the labors of Mr. Common, which have 
contributed more or less directly to the importance 
of his results. 
He began celestial photography about ten years 
ago, with a small refractor of five and a half inches 
aperture. In 1877 he supplied himself with an eigh- 
teen-inch mirror by Calver, the mounting for which 
was designed by himself, and executed under his 
direct personal superintendence. In a paper pre- 
sented to the Royal astronomical society in 1879, he 
laid down certain assumedly proper conditions to be 
fulfilled in the mounting of large reflectors, according 
to which he was proceeding with the construction of 
an exceedingly powerful telescope, and among which 
were the following: — 
1°. No tube properly so-called. 2°. No mass of 
metal either below or at the side of the line join- 
ing the large and small mirrors. 3°. An equatorial 
mounting capable of direction to any part of the visi- 
ble heavens, and of continued observation past the 
meridian without reversal. 4°. An efficient means 
of supporting the mirror without flexure. 5°. Driv- 
ing-clock, circles to find or identify an object, and 
motions taken to eye-end. 6°. A collimator for the 
ready adjustment of the mirrors. 7°. Such a con- 
struction of mounting as to give the greatest amount 
of steadiness with the least amount of friction. 8°. 
An efficient means of resilvering the mirrors and of 
protecting them from dew. 9°. A safe, steady, and 
easily adjusted platform for the observer, allowing 
about two hours continuous observation without the 
necessity of any motion except that from the observ- 
er’s place, and of easy access. 
In designing a mounting to satisfy these conditions, 
Mr. Common made such departures from the old 
form of mounting and platform, that an account of 
it was deemed worthy of a place in the Memoirs of the 
Royal astronomical society, where may be found (vol. 
xlvi. p. 173) a description of his instrument, together | 
with fully detailed drawings suited not only for his, 
but also for a much larger telescope. In the actual 
construction of the thirty-six inch reflector, the cost 
was kept down as much as possible without sacri- 
ficing any essential points, all elaborate mechanical 
SCIENCE. 
They are remarkable for the great dispar- 
(Vor. TIL, No. 65. 
arrangements coming under the head of mere luxu-_ 
ries being avoided. Both the telescope and its house 
were so contrived as to be completely under the 
management of one person. 
The difficulties which Mr. Common surmounted in 
the construction of his telescope were of the most 
discouraging nature, —in fact, unique. Just as the © 
great speculum —a lump of glass of about thirty-eight 
inches diameter, and seven inches thickness — was 
ready to receive its final figure in the hands of the 
optician, it burst into a thousand pieces with a terrific 
explosion. Within a few hours time, Mr. Common 
had telegraphed to the glass-makers in Paris for two 
more disks of like dimensions, the extra one to be 
brought into service in case of another explosion. 
The second disk, however, was successfully ground, 
polished, and mounted ready for work, about the 
middle of 1879, and it is with this instrument that Mr. 
Common has carried on his unequalled researches. 
In some respects it is proper to call it the most power- 
ful telescope in existence, although the great refrac- 
tor of thirty inches aperture, now being mounted 
near St. Petersburg, may be expected to surpass it. 
A description of Mr. Common’s novel plan for sil- 
vering large mirrors may be found in vol. xlii. of the 
Monthly notices of the Royal astronomical society, 
at p. 79. 
Of the mounting of Mr. Common’s reflector, Mr. 
Stone remarks, that it shows in every direction great 
engineering-skill, guided by the experience, gained in 
the use of the smaller instruments, of the actual re- 
quirements for successful astronomical work. The 
method of relieving the friction of the polar or main 
axis of the instrument deserves especial attention, 
and is fully dealt with in his memoir. Mr. Common 
alluded, in this publication, to the fact that this 
principle is equally applicable to other astronomical 
instruments of large dimensions; and at the meeting 
of the Royal astronomical society, March, 1884, he 
presented plans for a large transit circle in which 
mercury-troughs are used to sustain the weight of 
the tube when in certain positions. By these means 
he believes that flexure may be practically elimi- 
nated. 
Early in 1880 Mr. Common attempted to photo- 
graph the great nebula of Orion; the result being a 
failure, as the stars appeared on the plate as lines, 
and the nebula had impressed itself only as a faint 
stain. But such failures only suggested the neces- 
sity of improved clock-driving, and the use of more 
sensitive plates. In June, 1881, Mr. Common obtained 
a successful photograph of comet (b) of that year; and, 
in March of the year following, a photograph of the 
nebula of Orion, which excited the admiration of all 
the astronomers who had an opportunity of inspect- 
ing it. He continued, however, to push the refine- 
ments of his photographic and instrumental equip- 
ment to a farther limit, and obtained on the 30th of. 
January, 1883, a photograph of the nebula, with an 
exposure of thirty-seven minutes, a carbon enlarge- 
ment from the negative of which was presented to 
the Royal astronomical society in the March follow- 
ng. This photograph showed a marked advance on 
