848 
Reduction pause. Until very recent times, with few exceptions, 
of observa- our observers, however industrious, have merely 
pret used mechanically the apparatus put into their hands 
. ” by intelligent and conscientious artists, who yet, 
never having occasion to apply their own handiwork 
to the purposes for which it was made, could not be 
expected to detect deficiencies of construction, nor 
to possess the mathematical knowledge required to 
MATHEMATICAL AND PHYSICAL SCIENCE. (Diss. VI. 
wich in 1750 and some following years. In reality, —redue- 
no better observations had been then made. The in- tion of _ 
struments had undergone no material change during — 
the long and industrious, though not splendid, career tions, 
of Maskelyne; and Bradley was beyond question the 
most accomplished astronomer of the 18th century. 
Let us record it as a fact for the encouragement of 
conscientious labour in the service of science, that 
remedy them. To determine in a complete and inde- 
pendent manner the errors, not only of adjustment, 
but also of construction of the instruments, and to 
correct them by calculations (deduced when neces- 
sary from frequent observations of their amount) is 
now felt to be the duty of the astronomer himself, 
This improvement (though, as we shall see, already 
partially practised even in England) was mainly due 
to Bessel, whose name we have placed first at the head 
of this section. But astronomy owes to him much 
these precious records of the state of the heavens, of 
which the value seemed unknown to a whole succeed- 
ing generation (so that they were not even printed 
in a crude form until nearly fifty years after they 
were made), were disinterred, so to speak, by an 
illustrious foreigner, and in course of time made the 
basis of what, in respect of precision and method, 
might be called a new astronomy. Bessel was as- 
sisted in this work by a grant from the British Board 
of Longitude. 
more than this. He first showed that it is part of 
an intelligent astronomer’s duty, not only to observe 
stars and planets, but to undergo the vastly greater 
The observations especially considered by Bessel (223.) 
are those of the fixed stars. To apprehend their‘im- Special ob- 
portance, we must recollect that the foundation of an 1ects of = 
labour of comparing his results with the best theories 
of his time, and of improving as far as possible those 
theories, so as to hand over each branch of science to 
his successor in a more perfect state than that in 
which he found it. In all these respects practical 
astronomy commenced a new epoch in Germany, and 
on this account principally we place the name of 
Bessel in a conspicuous rank, 
(221) Frieprich WILHELM Bessex was born at Minden 
Bessel—his on the 22d July 1784. He was destined for a mer- 
“eek cantile career; and itis an interesting fact that it was 
the expectation of an appointment as supercargo on 
a commercial voyage which led him to study naviga- 
tion and astronomy, and finally induced him to de- 
vote his entire energies to the latter science, in which 
he received every encouragement from the amiable Ol- 
bers, who fortunately inhabited Bremen, where young 
Bessel was engaged in business. He commenced his 
astronomical career by reducing Harriott’s observa- 
tions of the comet of 1607, and was thereafter ap- 
pointed assistant to Schréter at the observatory of 
Lilienthal. In 1810 he was removed by the Prussian 
government to the charge of the observatory about to 
be erected at Kénigsberg, where he spent the re- 
mainder of his life, and which he very speedily raised 
by his labours to almost the first rank of European 
observatories. In fact, he possessed, in a singular 
degree, the qualifications for directing a great obser- 
vatory ; including a thorough acquaintance with the 
use of instruments, with the theory of astronomy in 
all its branches, with the higher mathematics, and also 
the art and practice of calculation, as well as with 
many allied branches of natural philosophy. 
(222.) The work by which he is perhaps best known, the 
His dopa Fundamenta Astronomia, is not grounded on his own 
rons ig, Observations, nor even those made in the same cen- 
tury, nor in his own country, but, what may appear 
singular, upon the observations of Bradley at Green- 
accurate knowledge of the orbits, and of the pertur- 
bations of the Solar System, not to mention less ap- 
parent but not less remarkable changes of place in 
the stars themselves, consists in rendering perfectly 
comparable observations of position at one time with 
those made at another more or less remote. Now, 
not only are our transit instruments and quadrants, 
and circles, and clocks, affected with errors more 
or less unknown, so that the registered figures re- 
quire correction; but, in consequence of Refraction 
and Aberration, we see no body whatever exterior 
to the earth in its real visual position; and this 
visual position, when found, is affected by Parallax. 
Besides all this, the grand points of reference in 
the sky, the Pole and the line of Equinoxes, are 
undergoing perpetual, though small, changes of po- 
sition :—the first star of Aries had already, in the 
time of Hipparchus, wandered from the point of in- 
tersection of the Equator and Ecliptie;—consequently, 
longitudes and right ascensions have to be reckoned 
from a directional line in space altogether imaginary. 
The pole of the heavens wanders amongst the stars, in 
consequence of Nutation—the obliquity of the Ecliptie 
is itself changing; and, to crownall these causes of per- 
turbation and seeming unsteadiness, the stars called 
fixed have (as we have formerly seen) peculiar and 
individual displacements not accounted for by any of 
the preceding causes of change, In short, nothing is 
fixed and comparable, except our measure of time. 
The length of the sidereal day is subject to no secular 
variation. The bases of Astronomy,—the Funda- 
menta Astronomie,—then, evidently consist in the 
perfect evaluation of all these varying elements, so 
that the stars, if not really fixed, may become, for us 
at least, virtually so,—that by their aid the position 
of the fundamental directional lines of the sphere 
may at any time be found, and the relative positions 
of the stars themselves. After this it is easy to 
