Donovan — O^i a ConijKirahle IlygrometeA'. 489 
and never more tight than is necessary to keep it in its rectilinear 
position : if the index, when moved, vibrate much and very rapidly 
the compound line is too tight. 
Old gut-lines act sufficiently well for hygroscopic purposes; but 
for exact experiments a new gut line should be used ; it is easily fixed 
in, and the cost of a number of them is almost nominal. The silk line 
need scarcely ever be renewed. 
When exact results are required, the natural zero should be the 
starting point ; in other cases, any degree may be used for that pur- 
pose. 
It is proper to mention, although it might have been anticipated, 
that this hygrometer scarcely acts in very frosty weather. When aqueous 
vapour is converted into icy particles, it is not in condition to be either 
absorbed or liberated: hence the gut line, although having an affinity 
for water, has none for ice. In continued hard frosts, the gut line is 
even permanently injured by tension and rendered unfit for service, so 
much so, as to be no longer affected by damp or drought, although 
originally moveable by the breath or the proximity of warm hands. 
The hygrometer being, as its name indicates, the measure of moist- 
ure, that degree at which its scale commences is therefore the natural 
zero; but throughout the foregoing description, the word zero is for 
shortness used to signify the completion of any round of the circle, that 
is any arrival of the index at the hundrdeth degree of the graduated cir- 
cle, which may occur ten times while its thousand degrees are being 
completed. The natural zero can be practically attained only by ex- 
posure of the gut line to the influence of exsiccants until the index 
stand motionless, which may not take place for several hours. 
It would be a depreciation of the value of this little instrument 
were it viewed as a philosophical trifle. It may be made conducive to 
health and comfort, and useful for a variety of purposes. In warm 
climates the atmosphere at times becomes so dry and parching that it 
is found necessary to ameliorate its effects by placing vessels of water in 
the most airy parts of the house, and in bed-rooms, and even on warm 
stoves in order to promote evaporation. The opposite condition of re- 
dundant moisture in the air is not without inconvenience — " We have 
(says Ganot) the feeling of oppression, even at moderate temperatures, 
when we are in an atmosphere saturated by moisture in which no 
evaporation takes place." In either of these states of the air the hy- 
grometer points to the evil and to the remedy. 
The instrument is available for ascertaining the dampness of new 
buildings or apartments, or the suitableness of the external air for vale- 
tudinarians. It may prove useful to the chemist, its small size rendering- 
it adaptable to his purposes. The air of cellars may be tested as to its fit- 
ness for the storage of provisions, wines, &c. It is perhaps the only 
manageable hygrometer for aeronauts, no preparation at the time of 
observation being necessary. It is convenient for comparing the air of 
mountains and valleys or widely separated localities, each of two obser- 
