490 
Proceedings of the Royal Irish Academij. 
vers being provided with a gut line from the same string. It shows 
how distant or how near is the point of liquefaction of vapour in the 
air, and thence the probability of a fall of rain, information so import- 
ant to the gardener or agriculturist. 
When a register is to be kept, during absence of a few hours in wet 
weather, or of many hours or even days in dry weather, the gut line 
must be previously brought to the maximum of dryness by means of 
the exsiccation discs, and the index brought to zero. A spherule is to 
be placed on each side of the index. On the return of the observer, if 
he find both spherules in the receiver, he is not necessarily to infer 
that damp continuously accumulated in the air to that amount, without 
intermission : there may have been advances and retrocessions of the 
index, the former having been, on that occasion, greater than the latter, 
until the spherules had been finally pushed through the perforation. 
The spherules and index together give the maximum, up to the moment 
of observation. 
A method of bringing the index to zero has been already given, but 
it is uncertain and only approximate. In order to bring the index pre- 
cisely to zero after being thus approximated, the tightening pin will be 
found to have complete control, but should not be used to great extent, 
as it affects the sensibility of the gut line. 
To learn the state of the atmosphere, at any time, the indication of 
the index will be generally insufficient : it may have previously tra- 
versed the circle several times by alternate absorption and extrication of 
moisture by the gut line, and of this the index gives no account. The 
amount in degrees, indicated by the spherules found in the receiver, if 
any, must be added to the degrees shown by the index. The sum is the 
present hygrometric state of the atmosphere. 
As the proper situs of a hygrometer is the open air, it must be pro- 
tected from vicissitudes of weather. It may be inclosed in a pocket 
lantern, consisting of three sides of glass closely glazed in a japanned 
tin frame, the fourth side being a door with many air-holes at the 
bottom. The top of the lantern may be constructed, in the usual way, 
with two arches of tin plate crossing each other at right angles. Hand- 
some pocket lanterns of this kind are commonly sold, but the open 
arches at top should be covered with muslin. 
The lantern, containing the hygrometer will, during rain and high 
wind, require to be included in a cubical tin case, so much larger than 
the lantern as will allow the entrance of a sufficient current of air 
through one side, which is to be left open. The case should be loaded, 
at bottom, with lead. 
The hygrometer should stand in a circular shallow tin tray, the 
bottom of which is covered inside with a disc of cloth to prevent the 
rebounding of the elastic spherules when they fall. This is the receiver 
previously mentioned. It shows at a glance how many spherules have 
fallen. 
During the late rainy weather I made a number of trials of my 
