556 
Proceedings of the Royal Irish Academy. 
LY. — On some Impeoyements of the Compaeable Self-acting 
Utgeometee, which kegistees the Maximum and Minimum of 
Humidity and Siccity of the Atmospheee in the Absence of an 
Obseryee. By M. Donoyan, Esq. 
[Eead 13th April, 1874.] 
Since my last communication was presented to the Academy, I have 
endeaYonred to remedy a deficiency in the instrument therein de- 
scribed. It registers the first two rounds of the index over the 
graduated circle, but does not register, although it measures the 1 
rounds which the index may subsequently perform. I now supply 
that deficiency. 
The exsiccated gut-line of the hygrometer when in its place, one 
end fixed motionless by the clamp- screw, and the other at liberty, 
will begin to move the index in damp air^' with its maximum motive 
power. Eut this force acts with decreasing efi'ect the nearer the 
acting portion of the line is to the fixed point where it is null. 
From this consideration, it appeared that if several indexes were 
affixed to the gut-line, at different heights from the dial, all pointing 
in the same direction, they would, in revolving, all point differently, 
and would complete their revolutions in different times. This is 
actually the case, as I found by a confusing experiment made with 
five paper indexes fastened at equal distances on the gut-line. 
Prom these facts I inferred that an index could be contrived 
which, besides moving over the graduated circle in the usual manner, 
might be adjusted to any position, higher up on the gut-line, without 
obstructing or interfering with the indications of the whole line, and 
that thus results might be obtained analogous to those exhibited by 
the hour-hand and minute-hand of a watch. 
I effected this object by attaching a secondary index to the gut- 
line in such a manner as to permit the latter, while twisting and 
untwisting throughout its whole length, -to turn both indexes at once, 
but at the different rates which belong to the two places on the gut- 
line to which they are attached. Ey such means, would be obtained 
movements similar to those of the hands of a watch. 
Such a secondary index may be easily contrived : let a disc of 
that substance called India-rubber sheet ^''^ about a quarter of an 
inch in diameter, and a tenth in thickness, be perforated through its 
centre by a sewing needle a little thicker than the gut-line intended 
* During wet weather accompanied by high winds this hygrometer, as well as 
others formed of animal or vegetable substances, will be but little affected. 
