284 PROCEEDINGS OF SECTION E. 
off by an auxiliary telescope. As the adopted arrangement 
could not well be understood without a diagram, I refrain for 
the present from entering into a description of the same. 
APPARATUS FOR MEASURING THE DISTANCE BETWEEN THE 
KNIFE EDGES OF THE PENDULUM. 
The essential difference between this and the one described by 
Captain Kater (Phil. Trans. 1818) consists in measuring the 
pendulum while being suspended. Great objections having 
been raised against the measuring of the pendulum in a hori- 
zontal position, as it cannot be supposed that it retains exactly 
the same leneth, even when stretched by weights attached to 
one of its extremities, I devised an apparatus which admits of 
the measuring of the pendulum in its vertical position, and at 
the same time affords the greatest possible accuracy. The 
brackets by which the pendulum is suspended are fixed to a 
piece of brass, which by the aid of a micrometer screw can be 
moved sideways, keeping both knife-edges under their respec- 
tive microscopes. By this arrangement different points of the 
knife-edges can be brought in contact with the wires of the 
microscope, and the means of the various measurements thus 
obtained will give the distances of the knife-edges, free of any 
error which may arise from any want of parallelism in the 
same. The setting scale and the misroscope and micrometer 
are very much the same as used by Sir George Shuckburgh in 
his experiments for fixing a standard for Great Britain (see 
Phil. Trans. for.1789), and it is only to be added that the 
micrometer has a magnifying power of 20 times, and that it 
admits of a reading of 0. 00166 of a millimetre. The standard 
scale or bar is a copy of the one used by Professor Bessel, and 
is divided in single millimetres, and as this excellent observer 
had taken great pains to compare his standard measure with 
the Toise de Perou, we may consider the one forming part of the 
apparatus used by myself as deserving all confidence. The same 
has been made and divided in Hamburg by T. Lohmeier, while 
the rest of the measuring apparatus is from the workshop of 
W. Henry Schreiber, of this city. It remains yet to be men- 
tioned that the whole is screwed to a cedar block, covered by a 
glass case when not used, and the instrument is fixed in a perpen- 
dicular position, which has from time to time to be verified, to 
the wall, only 2 ft. distant from the case of the pendulum. 
MopeE or ComMpPaRrISON OF PENDULUM AND Brass Bar. 
Of special importance is the mode of comparison of the length 
between the knife-edges and of the brass bar, because thereby 
we obtain the possibility of judging the agreement of the deter- 
mination executed by me in Melbourne and those by Professor 
