286 On a Method of fixing a Transit Instrument 
difference between the observed and apparent time of the 
transit of y Lyr@ is (18°. 52’. 37%,3—18". 52’. 9”,8) = +27",5: 
the deviation of the transit instrument has been found to he 
—38”,75 in time, and the number in the table, against N (=32°) 
is ‘39: the product of these two is —1",5: so that 27”,5 
—1”,5=26",0 is the error of the clock at the time of observa- 
tion, which being positive shows that the clock was too fast. I 
shall here repeat that the observed dzme, here alluded to, is sup- 
posed to be sidereal time: and therefore if mean solar time be 
employed i in the observation, it must be converted into sidereal 
time, by any of the methods laid down for that purpose. It 
may be useful to remark that, in all observations of this kind, it 
is presuined that the proper adjustments of the transit instru- 
ment are made previously to observation: and particularly that 
the axis of the telescope is rendered perfectly level: otlierwise 
the observation will partake of the error arising from this source, 
and render a further correction necessary. 
I shall conclude by observing that M. Delambre prefers this 
mode of adjusting a transit instrument to that of observing the 
passage of the circumpolar stars, which requires an interval of at 
least 12 hours, during which time considerable alteration may 
have taken place in the rate of the clock ; and therefore cannot 
be conveniently practised except when the days are very short, 
and in a building constructed peculiarly for meridional observa- 
tions. Whereas the observations, here alluded to, may fre- 
quently be completed in a few minutes; at all times of the year ; 
and often by daylight. ‘The tables are very easily computed, 
and therefore every practical astronomer who requires greater 
accuracy should calculate them for the latitude of his own ob- 
servatory. In which case, the labour will be very considerably 
abridged if he confines the table to the declination of those stars 
which are most frequently used by him for such comparisons, 
It may be proper to state, that the values in this table (except 
those in the last line) must be multiplied by the following num- 
bers, for any other parallel of latitude to the southward or north- 
worth of Greenwich: viz. if 
south 1°, by .979 north 3°, by 1.072 
north 1°, by 1.023 4°, by 1.099 
2°, by 1.047 
so that in no part of England will the correction amount to 
asth, nor if within 2 degrees of the latitude of Greenwich, will 
it amount to zth of the whole value. ‘The last line, for the 
correction of the clock, is adapted to the latitude of Greenwich : 
only. 
Sum 
