486 
Proceedings of the Royal Irish Academy. 
Two hygrometer frames of similar construction and size were pro- 
cured. A gut string, the thinnest, most even, and most equally 
twisted was selected ; and, from the middle of it, a piece twelve inches 
long was cut and divided into two equal parts, which weighed 0'54 of 
a grain each. A string vs^as made of a few fibres of floss silk, very 
loosely twisted together, divided into two, and a sufficient knot made 
on the end of each. The gut and silk lines were connected by the 
brass cones to form two compound-lines as already described, and one 
of these with its index was fixed in each hygrometer frame, care being 
taken that the same length of gut was exposed to the action of the air, 
which was easily effected by the sliding stem. The compound lines, 
hanging loosely, resumed their natural twist, and in this state were 
left for several hours. Each was then gradually tightened until the 
rapidity of the vibrations of the index of each, when moved equally 
from its position of rest and suddenly let loose, was equal in both ; and 
of this the eye seems to be a sufficient judge. The more rapid the 
vibration the greater the tension. The weakest tension that will give 
vibrations, and carry the index in both cases equally, is the proper 
condition of the two hj^grometers for comparison. It should be 
stated that one of these instruments had a silk string twice as long 
as the other. 
I shall now state the results of a comparison of those two hygro- 
meters, placed closely beside each other in the open air, and under 
shelter, during eight days. In this period, I inspected them 119 
times, at unequal intervals, while the indexes traversed the dials ten 
times round — backwards sometimes, forwards other times — but in 
effect ten times. In the following statement, no case is recorded 
wherein the indexes remained at the same degree as at the preceding 
inspection : — 
In 15 inspections the hygrometers agreed precisely. 
In 18 inspections they differed one degree. 
In 14 inspections they differed two degrees. 
In 4 inspections they differed three degrees. 
During these 51 inspections, the indexes had gone round the dials 
six times. But during the remaining four rounds the differences were 
much greater, reaching 20 degrees, and once 25 degrees. This might 
have been foreseen; for as each new round of each index gave its silk 
string an additional twist, and as one of the silk strings, being but 
half the length, offered double resistance to its gut line, the latter was 
at length nearly overpowered, and moved its index more feebly and. 
slowly, while the unobstructed one moved nearly at its original 
rate : hence the increasing difference between. There may have been 
many other cases of agreement not here recorded ; and but for this 
impediment it is fair to presume that there would have been agreement 
throughout the whole scale. The movements were very numerous, 
the weather being stormy and changeable. 
