Donovan — On a Comparable Hygrometer. 481 
roll round it, or obey any impulse communicated to it by the index. 
It will remain at some degree, and thus mark where the index stood at 
a particular time. Or, if a spherule be placed at each side of the index, 
the weather being damp, one will almost immediately begin to remove 
from the other, and will continue to remove until it reach the maximum 
of the time, or until its course be altered or stopped by some new 
atmospheric change : the first will show the condition of the air 
when the change commenced ; the second the angular amount of the 
change. 
To enable the index to move the spherule, it is provided with a pin 
at the end pointing downwards into the groove of the dial, but not so 
as to touch the groove. 
The rotation of the index would inevitably sooner or later throw 
these light spherules out of their proper place, or off the dial altogether, 
but for the prevention of the shallow groove in which they are placed. 
There are seasons when long continued rains render the atmosphere 
so loaded with moisture that the circle of the dial and its hundred de- 
grees would prove insufScient to record its changes. The index having 
traversed the entire circle, it may be inquired what can it do more 
than go round again and again ; and how is the hygroscopist to learn 
how often it has done so in his absence? Means are provided. Adjoin- 
ing that part of the circular groove marked 0 or zero is a perforation in 
the dial wide enough to allow a spherule to pass through it. When the 
spherule on duty has been chased from its first position beside the zero, 
all round the circular groove, it drops through the perforation into the 
receiver underneath, and its absence from the dial indicates that one round 
(or 1 00 degrees) hasbeen accomplished. The index proceeds on its course ; 
and if a second spherule had been placed beside the first, as should 
generally be, with the index between the two, the perforation being 
beside both, this second spherule will now be chased like the first, and 
will drop through the perforation, thus indicating the completion of a 
second round of the dial, or 200 degrees. The index, still proceeding, 
will at length pass over and beyond the perforation, and thereby indi- 
cate a third circuit, or 300 degrees, which however is not otherwise 
marked than by the passage of the index over and beyond the perfo- 
ration. Any degrees beyond this, to which the index may have arrived, 
short of zero, is a decimal of a fourth round. If a further account be 
required to be kept, two new spherules may be placed beside the index 
as before. More than this there is almost never occasion for, as such 
persistent moisture of the air is rarely experienced, without an oppor- 
tunity of resetting the instrument. 
When no account of consecutive rounds of the index is required to 
be kept, the hole for the fall through of the spherules might be incon- 
venient. To meet this, there is a slightly conical brass stopper fitted 
to the hole so exactly, its surface being made to constitute a portion of 
the groove, that not the least obstruction is opposed to the rolling of 
the spherules. 
