480 
Proceedings of the Royal Irish Academy. 
haps it has been long raining, For effecting this purpose conveniently, 
two or three discs of spongio-piline (a substance well known to the 
medical profession), of such a size as will ht in the glass receiver, are to 
be soaked in hot concentrated solution of chloride of calcium, and per- 
fectly dried before a fire. These are to be kept for occasional use care- 
fully secluded from the air: they may sometimes require to be r€ dried. 
When the effect of air of extreme siccity is required for a hygro- 
metrical experiment, these discs are to be laid on the glass plate, covered 
with a disc of card-paper, the hygrometer placed on the card, and the 
receiver, its edge slightly greased, over all. The motion of the index 
will immediately retrograde through 90°, 80°, 70°, &c. 
Should an atmosphere of extreme moisture be required, several discs 
of blotting paper, soaked in water, are to be substituted for the saturated 
spongio-piline: the motion of the index will be through 10°, 20°, 
30°, &c. 
There is a fact deserving of notice here, which, if unattended to, 
might lead to misconception in the management of experiments with 
this hygrometer. When the gut line has been dried to its apparent 
maximum, by means of chloride of calcium, if the receiver containing 
the hygrometer be immediately placed in hot summer sunshine, the 
index will indicate increasing dryness, to the extent of 40 or even 
60 degrees. This fact might lead to a doubt of the adequacy of the 
chemical means made use of for absorbing moisture, bat it may be 
explained by attributing the appearance of increased dryness to in- 
creased twisting of the gut line by heat, without any extrusion of 
moisture. Be this as it may, the exsiccation will give reliable 
results if performed in the shade. Winter sunshine has almost 
no effect. 
The degree indicated on the dial would give but little information 
unless viewed in conjunction with the previous situation and movement 
of the index : it is therefore necessary that there should be found on 
the dial some assistance to memory, besides the degrees. For this pur- 
pose, I use very small spherules, which are easily moved by the index 
during its progress. They must be perfectly smooth and light, in 
order that they may roll with the slightest touch : glass is a good 
material, and we have markers, almost ready-made, in those small black 
beads used by ladies for ornamental purposes : their size should be such 
that 100 shall average 16 grains weight. But they have two defects; 
they are perforated, and are in some degree flat, and therefore will not 
readily roll under a weak impulse. Both defects m^y be remedied by 
the following process. If one of these beads be placed on a piece of 
charcoal, and a jet from a voluminous flame be thrown on it, by means 
of a blow-pipe, it will speedily melt, and if thoroughly melted will, by 
cohesion, become a nearly perfect spherule; at the same time, the hole 
closes. When about to be used, both the spherule and the groove must 
be perfectly clean. 
A sphernle thus obtained, if placed in the groove of the dial, will 
