AND OTHER METEOROLOGICAL INSTRUMENTS, BY PHOTOGRAPHY. 
71 
*A continuous registration of the variations of the thermometer ha§ been obtained 
by intercepting the focal line of light formed on the paper as above described, by the 
stem of a thermometer having a wide flat bore. A sufficient quantity of light passes 
through the empty portion of the bore to darken the paper, but is entirely excluded 
from the portion occupied by the mercury. The register therefore consists of a light 
and a dark space, separated by a well-defined boundary line, the distance of which 
from the base-line will furnish the required indication. This particular application 
of the apparatus prefers no claim to novelty, as a very similar means of registering 
the variations of the thermometer has already been published*, and is here introduced 
merely as forming a necessary part of a complete system of automatic meteorological 
registration. 
As a thermometer with a large bore, and a scale sufficiently open to give the indi- 
cations of change with the requisite degree of minuteness, must of necessity contain 
a large quantity of mercury, which if contained in a globular bulb would not be 
sufficiently sensitive, the instrument which has been used has a long narrow tubular 
bulb (see Plate VI. fig. 11), by lengthening which any required amplitude of scale 
may be obtained without any diminution of sensibility. 
As small differences of temperature have a much greater influence at low tempera- 
tures in determining the hygrometric conditions of the atmosphere, from a compa- 
rison of the thermometer and psychrometer, than they have at higher temperatures, 
and as the range of variation is so much less during winter than in summer, it is 
proposed that a thermometer and psychrometer, having scales of about five degrees 
to one inch, should be used in winter ; while those for summer use should have a scale 
of about ten degrees to one inch. 
It having been found practically impossible to depend on the uniformity of a wide 
flat bore in a glass tube, a more than usually correct method of graduating these 
instruments would be desirable. This object would be attained by fixing to the stem 
a scale of -^ths of an inch, which by a suitable vernier might be read with any re- 
quired degree of minuteness. A separate comparison of the readings of this scale 
with two or more good standard thermometers should now be made, each being im- 
mersed with the instrument to be graduated in a vessel of warm water, which is 
allowed to cool very slowly. A mean of the results thus obtained would probably 
afford a very nearly correct graduation, which would, for the reasons above stated, be 
of most importance at low temperatures. 
If the register is required to furnish only differential results, a great length of stem 
may be obviated, and the safety of the instrument secured, if casually exposed to a 
temperature above its range, by a safety bulb at the upper end of the stem, as repre- 
sented in the diagram. By retaining a certain quantity of mercury in this bulb, the 
mean temperature corresponding to the time of year may be always made to occupy 
nearly the same place, and a change of thermometers thus rendered unnecessary. 
* See Engineers’ Magazine, Nov. 1845. 
