12 REPORT—1843. 
fact, that since the first discovery of the reflecting telescope by Sir Isaac Newton, 
every optician and amateur has blindly followed in the path laid down by that illus- 
trious philosopher, of polishing the mirror by rubbing it over the surface of a fixed 
polisher, the mirror being uppermost and the polisher under it. Lord Rosse was 
the first who ever tried the reverse of this process ; he placed the mirror de/ow, and 
moved the polisher over, its surface. To this simple change of position the doctor 
mainly attributed the uniform success of its illustrious inventor. By this change all 
the elements which enter into the one process are reversed in the other. In grinding 
the mirror on a metallic tool with emery, the perfection of that part of the process is 
to obtain a portion of a sphere, a figure known to every optician to be utterly un- 
suited to form areflector, in consequence of the zones near the margin of the mirror 
being too much curved in proportion to those nearer the centre, and that a parabola 
is the required curve. Now asmall portion of a circle can be changed to a parabola 
in two ways, either by increasing the curvature of the middle portion, or by diminish- 
ing the curvature of the extremes. In the former case the focus of a mirror so altered 
will be shortened, and in the latter case will be lengthened. It is found in polishing 
with the polisher undermost that the focus is shortened, while in Lord Rosse’s me- 
thod it is lengthened, which no doubt is the more simple and more certain mode of 
proceeding. 
On the regular Variations of the Direction and Intensity of the Earth's 
Magnetic Force. By Professor Luoyp. 
In this communication the author has given the principal results of the series of 
observations which have been made at the Dublin Magnetical Observatory, as far as 
they have been yet reduced. ‘These observations were commenced in the beginning 
of the year 1839, and have been continued, almost without interruption, to the present 
time. Since the beginning of the year 1840, they were taken every two hours, day 
and night, in accordance with the general plan of observation laid down by the Royal 
Society. The elements directly observed are the Declination and the two components 
(horizontal and vertical) of the Intensity; and from the variations of the latter those 
of the total Intensity and Inclination are readily deduced. The means of observing 
the vertical component of the intensity having proved not altogether satisfactory, an- 
other instrument has since been contrived by the author, by which the changes of the 
inclination are given directly. Professor Lloyd would not occupy the time of the 
Meeting with any account of the instruments or methods of observation, which are 
now sufliciently known by all interested in the subject of terrestrial magnetism ; but 
would proceed at once to the results obtained, so far as they related to the regular 
changes of the magnetic elements, commencing with the diurnal changes. These 
variations were projected in curves, which represented the course of the mean daily 
changes for the entire year, for the summer and winter half-years, and for each 
month separately. 
Declination.—The mean daily curve of the changes of declination, for the entire 
year, exhibits a small easterly movement of the north end of the magnet during the 
morning hours, which reaches its maximum about 7 a.m. After that hour the north 
end moves rapidly westward, and reaches its extreme westerly position at 1" 10™ p.m. 
It then returns to the eastward, but less rapidly, the easterly deviation becoming a 
maximum about 10 p.m. The mean daily range is 9:3 minutes, 
During the summer months the morning maximum at 7 a.m. is more marked; the 
evening maximum, on the contrary, disappears, there being a slow and regular move- 
ment of the north end to the eastward from 7 p.m. until 7 a.m. In winter, on the 
other hand, the evening maximum is well defined, and the morning maximum dis- 
appears, there being a slow and regular westerly movement until 9 a.m., after which 
the movement becomes more rapid in the same direction. The epoch of the extreme 
westerly position of the magnet is nearly the same throughout the year. The greatest 
daily range, in summer, is about 13°7 minutes; the least range, in winter, about 7-2 
minutes. 
Horizontal Intensity.—The mean daily course of the horizontal force, for the entire 
year, has two maxima and two minima. The first minimum occurs between 1 a,m. and 
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