482 
Proceedings of the Royal Irish A cademy. 
It is necessary that the position of the graduated dial-plate should 
be truly horizontal, as, if not, the spherules will indicate incorrectly. 
A wooden stand with three levelling screws would be convenient, if 
not indispensable. 
When it is desirable to observe the hj^grometrical state of the atmo- 
sphere and its changes more particularly, it will be proper to start from 
the natural zero, hereafter described, which point may be attained by 
the use of the ground glass receiver with its glass plate and discs im- 
pregnated with dry chloride of calcium. The index is to be brought to 
the hundredth degree by loosening the clamp-screw which confines the 
gut line at top, and turning the latter round until the index point cor- 
rectly : the gut line being then very moderately stretched, and the 
clamp-screw tightened, the hygrometer is ready for use. The tighten- 
ing of the clamp-screw generally removes the index a few degrees from 
zero, but a slight turn of the tightening-pin one way or the other will 
restore it to its place. 
While the hygrometer is in this state, should the weather be dry, 
the index will not at that time change its position, for it is already at 
its maximum of dryness. If the weather be wet or damp, a spherule 
placed in the groove will be gradually pushed round, according to the 
numerical increase of the degrees engraved on the dial, until it fall 
through the perforation. Dampness and dryness alternating perhaps 
several times during the day, the course of the index will be as often 
reversed until ultimately the spherule disappear, indicating that 100 
degrees have been traversed. 
At any time when it is desirable to know at what rate the atmo- 
sphere is becoming more or less dry, should there be any change, a 
spherule may be placed, touching the index at each side; and on the 
return of the observer, the separation of the two will give the desired 
information in degrees. In such cases, it will not be necessary to bring 
the index previously to zero. 
But an observer may then, or at any time require to know whether 
the index, now apparently at rest, have been advancing in dampness 
or receding in dryness. This is shown by a minute steel bead, at- 
tached by a flaccid fibre a quarter of an inch long to the balance-end 
of the indeXo This bead will trail after the index, and thus show the 
direction of the last or present movement. Eut for the steel bead, in- 
spection of the hygrometer would not show, at a glance, the present 
state or latest change of the index. The only sufficiently flexible fibre 
that succeeded in my very numerous trials was a bit of the warp of the 
finest Prench cambric, washed in distilled water, and softened by beat- 
ing : every other fibre tried, whether wool, cotton, or even cocoon silk, 
failed, being too elastic. When the steel bead is directly under the 
index, the weather is changing. 
It is necessary here to explain what I mean by advancing in damp- 
ness or receding in dryness : the former is meant when the index 
