of the Equatorial Instrument. 117 
respect to those of right ascension. It must be readily seen, 
that the amount of these errors will be pretty nearly the 
same as those of any other transit instrument, whose magni- 
fying power, and length of axis, are the same. However, from 
actual trial, I lind that the passage of a star, near the equator, 
over any one wire in the field of the telescope, may be deter- 
mined to within | of a beat of the regulator, in strictness to 
about 3", 7, and, from a mean of the three wires, to within 
1 ",25 of a degree, = ~ of a second of time ; that is, if the 
wind be still, the weather favourable, and reasonable care be 
taken. And, from a series of observations of the sun's 
diameter throughout the year, it appears, the error in ordi- 
nary observations may be expected to lie within 3"; that it 
is 17 to 1 that this error does not amount to f', = ± of a se- 
cond of time ; and this includes some of the worst weather 
in which observations are likely to be made. I shall there- 
fore say, that 
The probability of error of an observation of a transit 
over the meridian, under tolerably favourable circumstances, 
from a mean of 3 wires, viz. in estimating the beat „ 
of the regulator, is about - - =2,0 
Add to this the error of setting the instrument to 
the meridian mark - - - = 1,0 
Add, also, the error in reading off the equatorial 
microscope - - - - = 0,5 
Total error of an observation in the meridian 
becomes - 
To this add the error of the divisions, and of the 
centre (at most) - 
= 1,0 
