AND OTHER METEOROLOGICAL INSTRUMENTS, BY PHOTOGRAPHY. 
63 
given variation with sufficient accuracy. As it is extremely improbable that two or 
more cylinders should be obtained of precisely similar circumference, it is desirable 
that a correct time-scale should be constructed for each cylinder employed, which 
may be readily effected in the following manner. Let a piece of paper, made as 
damp as the photographic paper usually is when placed on the cylinder, be cut so as 
to exactly meet round it; when dry, let this be divided into twelve equal parts, 
each of which will represent l h ; these again into twelve parts, each of which will 
correspond to 5 m . By this arrangement an allowance is made for the shrinking of 
the paper*. The angular deviation is measured from a line on the paper drawn in 
the direction of its motion, such, for example, as would be described by the point 
of a fixed pencil resting constantly against it ; and at the scale of 0*05 inch to one 
minute (which may perhaps be taken as the best working scale with the existing 
apparatus), the edge of the line when the various adjustments have been carefully 
attended to, is sufficiently well defined (as may be observed by a reference to many 
of the photographs) to determine its place to half or less than half a division of a 
scale of ^gth of an inch, that is, the position of the magnet may be read to within 
ten seconds. The plan adopted for determining the position of the base-line on the 
photographs, has been that of drawing pencil marks across the line at which one end 
of the paper overlaps the other, previously to its removal from the cylinder. When 
the register is dried, a line drawn through the two corresponding marks at the two 
ends has been taken to be the base-line. As however it can never be expected to 
obtain glass cylinders that have either perfectly cylindrical surfaces or perfect sur- 
faces of revolution, the line of intersection of a plane perpendicular to the axis of the 
cylinder with the paper, would not probably be a perfectly straight line when the 
paper is opened out ; and if the paper should from any cause contract unequally in 
drying, this line of intersection, which is the true base-line, would be liable to some 
further distortion. A comparison of many photographs with the Greenwich observa- 
tions has led to the belief, that a small portion of error has occasionally arisen from 
these causes ; there is, however, no reason for supposing that it has ever exceeded 
one minute. The means of obviating these sources of error by a photographic base- 
line are under consideration, but have not yet been perfected'!'. 
* It is questionable whether the inequalities in length of the different registers, arising from different 
degrees of moisture and corresponding expansion of the paper, and from other causes, may not be too great to 
admit of the use of any fixed scale ; in this case it will be necessary to divide each base-line into twelve parts 
by a suitable pair of proportional compasses, or some other contrivance. The errors arising from unequal 
expansion of different parts of the same piece of paper would, it is believed, be too minute to be recognised. 
In order, however, to guard against these sources of error, and generally for the purpose of marking any given 
epoch on the paper, the pencil of light by which the base-line is described is shut off by a sliding piece attached 
to the cover. After remaining so for a few minutes, it is readmitted for a period of ten seconds, and again shut 
off for a few minutes more, the time of its readmission being recorded. A small dot in the midst of an inter- 
rupted portion of the base-line will serve to determine the given epoch on the register, for an example of which 
see fig. 8, Plate IX. — May 1847. 
t In the combined register of the declinometer and bifilar magnetometer, the base-line is now described by a 
