REPORT ON ASTRONOMY. 
165 
specting a comet which some observers conceived they had 
seen upon the sun’s disk. In 1826 M. Gambart found that a 
comet would cross the sun’s disk: he watched the sun most 
carefully at the time predicted, but nothing was visible. The 
dilatation of Encke’s comet as it receded from the sun, has given 
rise to some speculations on the nature of the ether pervading 
space. 
I may mention in this place that the method of minimum 
squares and estimation of probable errors, though applicable 
to almost all physical calculations, have been most extensively 
used in calculations for comets, and were in fact first proposed 
in treatises on comets (Legendre’s Ncuvelles Methodes, and 
Gauss’s Theoria Motus). I will not undertake to say that I 
think the method of minimum squares is unexceptionable in all 
its applications, or that I attach much more than a relative 
value to the estimation of probable errors . But I think there 
is no doubt that these methods have contributed much to the 
accuracy of modern astronomy, and that in many doubtful 
cases they have been admirable assistants to the astronomer’s 
judgement. 
VIII. The materials upon which a knowledge of the earth’s 
figure was grounded, at the beginning of the century, were the 
following. The arc measured in Peru by Bouguer, Lacon- 
damine, &c. ; that measured in Lapland by Clairaut, Mauper- 
tuis, &c.; that in America by Mason and Dixon, &c. ; that 
from Rome to Rimini by Boscovich ; and that from Barcelona 
to Dunkirk, measured by Delambre and Mechain. Besides these 
there were some others, as one in Piedmont by Beccaria, one 
in Austria by Liesganig, and one in India by Reuben Burrows, 
to which little credit was given; and there was Lacaille’s 
measure at the Cape of Good Hope, which could not be re¬ 
conciled with the others. One arc of parallel had also been 
measured in France : and one of much greater value in En¬ 
gland. The pendulum experiments (serving, with the help of 
Clairaut’s theorem, to determine the proportion of the earth’s 
axes,) were principally scattered observations by De la Croyere, 
Campbell, Mairan, Bouguer, Godin, Maupertuis, Lacaille, Le- 
gentil, Phipps, Malaspina, and Borda. The last of these (con¬ 
fined to Paris,) were the only ones from which great accuracy 
could be expected ; of the others, the only set in which a series 
of considerable geographical extent were observed by the same 
persons and with the same instrument, was Malaspina’s. The 
observations of the attraction of Schehallien, and Cavendish’s 
experiments with leaden balls, had given a pretty good know¬ 
ledge of the earth’s mean density. 
