TRIGONOMETRICAL SURVEY OF NEW SOUTH WALES. 217 
fact seldom is done exactly, and whenever the readings are not 
equally distant, on opposite sides of 20 R, a correction is necessary. 
In one case, an exceptional one certainly, it was found that the 
difference of zenith distance required correction on this account 
by as much as 12”. Although the effect of such errors would be 
reduced by oriulsixedteien of many pairs of stars, as it would be 
improbable that the error of screw would have the same sign 
with each pair of stars; still it is shown clearly enough that this 
is a source of error to be reckoned with, and it is one which will 
most likely increase. 
As to its cause there can be little doubt. Seeing the thousands 
of measures made with the screw since 1882, when it was last 
xamined, the wonder would be if by now the effects of wear 
were not to be seen, and it is probable that wear zs the cause of 
the different values given by different parts. If, as in the present 
case, a screw is more frequently used in one part than in another, 
and one side only of its thread is subject to friction owing to the 
constant action of a spring, it seems patent that the consequent 
wear must ultimately result in an appreciable alteration of the 
position of the thread at the more used part relative to the thread 
at the less used parts ; the interval on one side of the used part 
being increased in length, and that on the other side decreased. 
Such action would entirely account for the defect observed in this 
screw. The middle of the screw is most used ; and the interval 
from the middle to one end is found to have increased beyond the 
value it had in 1882, while the other end has correspondingly 
decreased. 
TEMPERATURE EFFECT ON MICROMETER. 
The values of the micrometer revolution, as derived from the 
various stars, are as follows :— 
From star 72, R=58"°629 From star 8914, R- 58”°672 
»» 6703, R=58""728 , 9273, R=58""806 
5 16380, R=58"'634 re 2452, R=58"'552 
The mean is R=58"-670. It is noted that though the theoretical 
probable errors of these values, arrived at from each individual 
set, are only + °007, +°006, +°"003, +-"001, +-’002, and +-”003, 
respectively, yet much larger differences are found between the 
above determinations. This points to a divergence of the con- 
ditions under which the observations were made, and it is to be 
regretted that in such observations the temperature of the 
instrument is not generally noted, in order that it might be 
ascertained whether that is a disturbing element. In delicate 
work of this kind some means should be found for ascertaining 
the temperature of the instrument, as it must change not only 
from the observer’s presence but from the effect of the actual 
