252 An EASY METHOD of FINDING THE 
this purpofe, requires the previous knowledge either of the 
latitude of the place, or hour of the day, at the time of 
obfervation; neither of which can, by the common ap- 
paratus of a furveyor, be found with fufficient accuracy. 
The fun’s azimuth may, it is true, be found without 
knowing either the latitude of the place or hour of the 
day, by taking equal altitudes before and after noon; but 
this requires time, attention and inftruments, which fur- 
veyors can but feldom command. 
That method, which is perhaps the moft exact, viz. 
meafuring the time between the paflage of two ftars which 
differ confiderably, in declination and but little in right 
afcenfion, over the fame vertical circle, is ftill farther out 
of the reach of common furveyors. 
The following table of the pole far will, it is prefum- 
ed, furnifh a more eafy, and yet fufficiently accurate me- 
thod of determining this problem; free from all the 
above inconveniencies, and requiring no difficult calcula- 
tion, nor any other inftrument than the common theodo- 
lite, or circumferentor. For though the latitude of the 
place fhould not be known within a whole degree, nor 
the hour of the night within 2 or 3 minutes, this table, 
by a fingle obfervation of the magnetic azimuth or bearing 
of the pole ftar, will generally give the variation of the 
needle true to a fingle minute of a degree. Nay if the ob- 
fervation be made (as it may be every night) when the ftar 
is near its greateft elongation, an error of 10, or even 20 
minutes in time will, as is plain from the table, produce lit- 
tle or no fenfible error in the azimuth. And as thefe obfer- 
vations may be repeated at pleafure during the night, and 
a mean of all taken, the variation may, by this means, be 
found-to any degree of accuracy that can be defired. Be- 
fides, the needle is ‘not at this time affected with any di- 
urnal variation; which in the day-time is very uncertain, 
and frequently amounts to more than one quarter of a 
degree. 
The 
