AND METEOROLOGICAL INSTRUMENTS, BY PHOTOGRAPHY. 
21 
a pulley at the point of suspension. Towards either end, the rod and tubes are con- 
nected by a moveable clamp, by which the two may be clamped togetlier at any 
required distance from the centre. It is evident that by elevation of temf)erature the 
free ends of the zinc tubes will be approximated to each other by a quantity equal to 
the difference of the expansion of the lengths of zinc and glass that intervene Ijetvveen 
the sliding clamps and the free ends of the tubes, and consequently that a diminution 
to the same extent of the distance between the lower ends of the suspension skein 
will take place. The variation of the interval between the lower ends of the skein, 
corresponding with any given variation of temperature, may be made to bear any 
required ratio to the whole interval, first by a due adjustment of the upper and lower 
intervals of the skein, and secondly by varying the position of the sliding clamps, 
that is, of the acting lengths of the expanding tubes : the former may be considered 
as a coarse, the latter as a fine adjustment. The glass rod rests on rollers attached 
to the under surface of the tubes opposite to the hooks, in order that no jerking may 
be occasioned by the expansion or contraction of the zinc tubes. By these means 
the quantity b in the preceding formula may be made to vary by change of tempe- 
rature, proportionably to the change of the quantity m with any required degree of 
exactness ; so far, at least, as the variation of m is directly proportional to the variation 
of temperature. 
In the adjustment of the instrument the following steps are necessary. First, let 
the temperature coefficient of the bar be determined by the method described in the 
Paper No. III. previously mentioned ; secondly, let the lower interval of the skeins 
be taken, such that the ratio of the difference of linear expansion of the whole length 
of the tubes and glass rod (which for convenience is made the same length as the 
bar), between 32° F. and any given higher temperature, say 92° F., to the distance 
between the threads, may be considerably less than the whole correction for that in- 
terval of temperature ; ample scope will thus be allowed for determining by experi- 
ment the requisite amount of compensation, as is the case in the adjustment of chro- 
nometers. 
The pulley over which the skein passes at the upper point of suspension being made 
of brass, there will be an increase of the upper interval between the threads, with in- 
crease of temperature : in order to compensate for this, it will be necessary to take 
the acting lengths of the compensator somewhat greater than the calculated length, 
expressing the value of the temperature coefficient. 
The instrument having been thus approximately adjusted, the magnet and its ap- 
purtenances are now to be enclosed in a rectangular jacketed zinc box. The water 
in the jacket may be raised to any required temperature, and the temperature 
maintained nearly constant for any required period, by heating a pipe connecting 
the inlet and outlet of the jacket by a jet or jets of gas. 
A uniformity of the temperature of the jacket is obtained by the introduction of 
suitable diaphragms to ensure a complete circulation throughout its entire extent. A 
