84 MR. BROOKE ON THE AUTOMATIC REGISTRATION OF MAGNETOMETERS, 
the two thermometric instruments to be registered by one apparatus and on one piece 
of paper, was at first open to a grave objection, which has however subsequently been 
entirely removed. The pencil of rays incident on the stem of the thermometer must 
necessarily be a fan-shaped pencil, by the oblique rays of which the points corre- 
sponding to each degree would not be transferred to the respectively opposite points 
of the paper. If the surface of the cylinder were always parallel to, and equidistant 
from, the stem of the thermometer, this distortion of the scale would be constant and 
uniform, and therefore readily estimated ; but from the unavoidable imperfections in 
form and variations in size of the different cylinders employed, it would be extremely 
difficult to estimate correctly the distortion of the scale, and hence to infer the true 
temperature from the register. And this uncertainty would have been especially felt 
at very low temperatures, when the place of the mercury is impressed by the most 
oblique rays on the paper, and when small errors of relative temperature would 
largely affect the deduced hygrometric condition of the atmosphere. This difficulty 
has been obviated by enabling the apparatus to print continuously the scale of the 
thermometers, as well as to indicate the position of the mercury. This has been 
effected by placing fine wires, opposite to each degree, across the aperture in the 
scale frame, through which the light is transmitted to the stem of the instrument. 
By these wires a minute portion of the exposed paper is protected from light, and 
thus the darkened portion of the register is traversed by a series of parallel lines, 
corresponding with the scale of the thermometer. In order to remove any ambiguity 
in the reading of this scale, a coarser wire is placed at every ten degrees, and an 
additional coarse wire at the points 32 °, 54 °, 76° and 98 ° ; as one of these points may 
always be made to appear on the register, the relative position of the extra coarse 
wire will determine the point of the scale which it represents. 
It may here be mentioned that the wet bulb, although more than 6 inches in 
length, is kept perfectly saturated by being moistened at three different points by 
small bundles of lamp-cotton placed round the muslin covering of the bulb, and 
immersed in a vessel of water placed nearly opposite its middle point. 
It is very evident that the apparatus must afford some ready method of marking the 
time-scale on the paper, that is, of identifying any given epoch of time with the indica- 
tions of the register : this is effected by closing at any two known times the sliding doors 
of the cylindrical case, for five minutes, and then re-opening them. Two undarkened 
lines will be observed on the paper, corresponding to the known times; the intervening 
space being subdivided by the elastic scale, the time-scale is rendered complete*. 
It may also be remarked, that in all the other photographic registers obtained at 
the Royal Observatory by the instruments previously described, the only certain me- 
thod of marking the time-scale is found to consist in breaking the continuity of the 
line at a known epoch : this is effected by a piece of brass similar to one side of a 
* As a facsimile of a photographic diurnal register will be found in the Greenwich Magnetical and Meteo- 
rological Obsenmtions for 1837, it is unnecessary to introduce it in this place. 
