Donovan — On the Comparahle Self-Acting Hygrometer. 557 
to be used ; and, the needle being removed, let the end of the gut- 
line be passed through the perforation. Then let a thin wire, hooked 
at one end and sharpened at the other, be pushed into the disc, at a 
right angle with the gut-line. The wire is to be of such length that 
the hook will project directly over the graduation of the circle. 
Erom the hook is to be suspended, by a loop, a piece of the finest 
spool cotton, with the smallest particle of lead at the end to secure 
verticity, the cotton being of such length that the lead, in revolving, 
can at all times pass over the primary index. This constitutes the 
secondary index, which may be occasionally used or laid aside : it 
represents the hour-hand of a watch ; the primary the minute-hand : 
its place is variable on the gut-line. 
India-rubber is well fitted for this use as it takes sufficient hold 
of the gut-line to secure its remaining at the required height, while 
it does not hold it so fast as to prevent its twisting or untwisting, 
and at the same time turning the secondary index round. In some 
states of the weather it loses, but recovers its power. 
By this arrangement, the secondary index will, independently of 
the primary, be carried round the graduated circle ; and its pendent 
particle of lead will point downward exactly to the degree on the 
dial over which the secondary index at that moment stands. If the 
gut-line be viewed from a lateral position such that it shall cover the 
cotton thread, the degree on the dial, at that moment cut by the line, 
will be exactly seen, provided that the instrument stand vertically 
on a truly horizontal plane. 
That the secondary index does really act in the capacity specified, 
and is turned by the gut-line, in damp air, according to the degree of 
motive power which belongs to the portion of the gut-line to which 
it is attached, is obvious from the fact that if it do not so act, it 
should turn with, and exactly at the same rate as the primary index, 
which it never does ; and that, in returning in dry air, it retraces 
retrogradely the degrees over which it had passed during its progress 
forward. 
I now proceed to show in what manner the secondary index keeps 
an account of the number of rounds (each 100°) completed by the 
primary during a lengthened period of moist weather. If the secon- 
dary index revolved at an equal rate with the primary, the required 
information could not be obtained; but they cannot revolve at an 
equal rate, for the secondary stands higher on the gut line than the 
primary, and it has already been observed that the motive force of the 
gut-line acts with decreasing energy the nearer the acting portion is 
to the fixed point where it is null, viz. at the clamp- screw. 
"When the length of gut-line exposed to damp air, from the 
clamp- screw which confined it, to the upper apex of the double cone 
was 2^1 inches, and the distance from the clamp- screw to the secon- 
dary index was \^ inch, the latter measure being included in the 
