12 REPORT—1848, 
«The instrument may be furnished with a graduated quadrant for the purpose of 
adapting it to any latitude; but if it be intended to be fixed in any locality, it may 
be permanently adjusted to the proper polar elevation and the expense of the gradu- 
ated quadrant be saved: a spirit-level will be useful to adjust it accurately. The 
instrument might be set to its proper azimuth by the sun’s shadow at noon, or by 
means of a declination needle; but an observation with the instrument itself may be 
more readily employed for this purpose. Ascertain the true solar time by means of 
a good watch and a time equation table, set the hand of the polar clock to corre- 
spond thereto, and turn the vertical pillar on its axis until the colours of the selenite 
star entirely disappear. The instrument then will be properly adjusted. 
«The advantages a polar clock possesses over a sun-dial are,—1st. The polar clock 
being constantly directed to the same point of the sky, there is no locality in which 
it cannot be employed, whereas, in order that the indications of a sun-dial should 
be observed during the whole day, no obstacle must exist at any time between the 
dial and the places of the sun, and it therefore cannot be applied in any confined 
situation. The polar clock is consequently applicable in places where a sun-dial 
would be of no avail; on the north side of a mountain or of a lofty building for 
instance. 2ndly. It will continue to indicate the time after sunset and before sun- 
rise; in fact, so long as any portion of the rays of the sun are reflected from the 
atmosphere. 3rdly. It will also indicate the time, but with less accuracy, when the 
sky is overcast, if the clouds do not exceed a certain density. 
“‘ The plane of polarization of the north pole of the sky moves in the opposite di- 
rection to that of the hand of a watch; it is more convenient therefore to have the 
hours graduated on the lower semicircle, for the figures will then be read in their 
direct order, whereas they would be read backwards on an upper semicircle. In the 
southern hemisphere the upper semicircle should be employed, for the plane of 
polarization of the south pole of the sky changes in the same direction as the hand 
of a watch. If both the upper and lower semicircles be graduated, the same instru- 
ment will serve equally for both hemispheres.” 
Several other forms of the polar clock were then described; the following is a 
description of one among them, which, though much less accurate in its indications 
than the preceding, beautifully illustrates the principle. 
On a plate of glass twenty-five films of selenite of equal thickness are arranged 
at equal distances radially in a semicircle; they are placed so that the line bisecting 
the principal sections of the films shall correspond with the radii respectively, and 
figures corresponding to the hours are painted above each film in regular order. This 
plate of glass is fixed in a frame so that its plane is inclined to the horizon 38° 32!, the 
complement of the polar elevation; the light passing perpendicularly through this 
plate falls at the polarizing angle 56° 45' on a reflector of black glass, which is inclined 
18° 13’ to the horizon. This apparatus being properly adjusted, that is so that the 
glass dial-plate shall be perpendicular to the polar axis of the earth, the following 
will be the effects when presented towards an unclouded sky. At all times of the 
day the radii will appear of various shades of two complementary colours, which we 
will assume to be red and green, and the hour is indicated by the figure placed 
opposite the radius which contains the most red; the half-hour is indicated by the 
equality of two adjacent tints. 
On rendering the Electric Telegraph subservient to Meteorological Research. 
By Joun Bart, MRA. 
What is popularly termed the weather is a general expression for the physical 
condition of the atmosphere with reference to heat, pressure, moisture, and the ve- 
locity and direction of its motion. Two classes of causes determine these conditions 
at any given point of the earth’s surface. The first class may for short periods of 
time be considered as constants, depending on the position of the point of observa- © 
tion on the globe and the physical conformation of the adjoining district. The 
second class, upon which the proverbial uncertainty of the weather depends, arise 
from the influence exerted by each portion of the atmosphere upon those surround- 
ing it, by virtue of which a disturbance of equilibrium at any one point is rapidly 
propagated in all directions. In common language this is expressed by saying that 
