154 Royal Society. 
ascertaining the best kind of steel for the construction of a compass 
needle, the most advantageous form to be given to the needle, and 
the most effective mode of communicating to it magnetism. Many 
curious and unexpected results were obtained in the course of this 
investigation. 
13. A remarkable volcanic appearance in the moon being observed 
by Capt. Kater in February 1821, he communicated to the Society 
shortly afterwards an account of the phenomenon, which was pub- 
lished in the Phil. Trans. for the same year. 
14. Oneof the greatest benefits conferred onscience by Capt. Kater 
was his invention of the floating collimator, an instrument of which 
the object is to determine the situation of the line of collimation of 
a telescope attached to an astronomical circle, with respect to the 
zenith or the horizon in any one position of the instrument; or in 
other words, to determine the zero-point of the divisions on the limb: 
an operation which was before usually performed by the use of the 
level or the plumb-line, or by the reflexion of an object from the 
surface of a fluid. Each of these methods was liable to many in- 
conveniences and defects; all of which are avoided in the floating 
collimator. The principles on which this instrument is constructed 
are two; the first is the property of a telescope employed by Gauss, 
and subsequently by Bessel, in virtue of which the cross wires of a 
telescope adjusted to distinct vision on the wire, may be distinctly 
seen by another telescope also similarly adjusted, at whatever distance 
the telescope may be placed, provided their axes coincide ; in which 
case the rays diverging from the cross wires of either telescope, will 
emerge parallel from its object-glass, and will therefore be refracted 
by that of the other telescope to its sidereal focus, as if they came 
from an infinite distance. The other principle, which is employed 
as a substitute for the common level, is the invariability with respect 
to the plane of the horizon, of the position of a body of determinate 
figure and weight, when floating on the surface of afluid. Thus the 
telescope being attached to a box floating on mercury, and serving 
as a stand to the telescope, may be fixed either in a horizontal or a 
vertical position; in which latter case the reverse observations may 
be made by merely turning the float half round in azimuth. 
15. The later improvements made by Capt. Kater in the vertical 
floating collimator are described by him in a subsequent paper pub- 
lished in the Philosophical Transactions for 1828. Besides ob- 
viating the sources of error arising from the necessity of transferring 
the instrument to different sides of the observatory, and of taking the 
float out of the mercury and replacing it at each observation, the 
vertical floating collimator has the further advantage of being adapted 
for use, not only with a circle, but also with a telescope, either of 
the refracting or reflecting kind. Such a telescope, furnished with 
a wire micrometer, and directed to the zenith, becomes a zenith 
telescope, free from all the objections to which the zenith sector, 
and the ordinary zenith telescopes with a plumb-line, are liable. 
From the greater degree of precision attainable by the employment 
of this instrument, from the facility of its construction, the readiness 
