AND METEOROLOGICAL INSTRUMENTS, BY PHOTOGRAPHY. 
85 
parallel ruler, placed edgewise, one of the connecting pieces being prolonged and 
passing down through the stand to act as a lever, by which the parallel moving piece 
is raised between the cylindrical lens and the cone of the cylinder, so as to intercept 
the pencil of light which traces the register line. The light is usually excluded for 
5™, admitted for I"", again excluded for 5'“, and then re-admitted, the times of exclu- 
sion, admission and re-admission being recorded. If, during the second passage of 
the tracing pencil of light over the paper, a break of six or eight minutes be made, 
without the intervening spot, it will be found a convenient method of distinguishing 
the two lines, in case of any ambiguity. 
The scales of the thermometers in use have about 8° to 1 inch, from the registers 
of which the temperature may be readily read with certainty to less than a tenth of a 
degree. Of this scale, a space of about 60° may be illuminated at one time ; and in 
order that the temperature indicated may always be within the field, the thermometers 
are capable of being raised or lowered by a screw, so as to bring the mean tempera- 
ture of the season nearly opposite the middle of the paper : thus there is no probability 
that the record of any unusual and extreme changes of temperature will be lost. 
In the description of the camphine lamp given in the first paper, previously referred 
to, the wick was stated to have been placed below the diaphragm in the chimney ; 
it has since been found that there is another position in which equally perfect com- 
bustion takes place, which is when the wick is raised to about an equal distance 
above the diaphragm ; with this position of the wick, the liability to smoke is very 
materially diminished. 
Equally good effects have recently been produced by gas, saturated with the vapour 
of coal naphtha, which renders the light much whiter and more intense. Without 
this addition to the gas the photographic paper is feebly affected during periods of 
rapid movement of the magnet. The light used is that of a small fish-tail burner, so 
made as to spread the gas as much as possible ; the flame is placed edgewise towards 
the mirror, in which position the illumination of the mirror is the brightest. 
The employment of these instruments is not however limited to localities in which 
either camphine or gas is accessible ; for the barometer and thermometers, in which 
large and rapid movements of the tracing’ pencil of light are never required to be de- 
picted, an oil-lamp will answer sufficiently well : and by the same means the ordinary 
diurnal variations of the magnetometers may be delineated ; but no opportunity has 
yet occurred of obtaining, by means of an oil-lamp, a register of the rapid move 
ments occurring during a considerable disturbance. 
On a New Method of determining the Scale and Temperature Coefficients of Magnets 
used in observing the Changes of Magnetic Force. 
It appears from the ordinary formula expressing the equilibrium of the bifilar mag- 
net, that small changes in the amount of horizontal force will have the same effect 
in displacing the magnet, as small corresponding changes in the suspended weight. 
Having then carefully weighed the magnet, the mirror and the suspending frame, two 
