THE USE OF THE MAGNETIC NEEDLE IN SURVEYING. 1 65 



plan or report of every survey, stating at what time and by which of 

 these meridians the decUnation was taken. 



In the absence of such meridians located at the public expense, the 

 surveyor may, with little trouble, determine one for himself. The remain- 

 der of this paper will be devoted to an explanation of some of the 

 methods by which the astronomical meridian of a place may be found. 



I. By observations upon the pole star (Polaris). This star has now 

 (Jan. I, 1874) a polar distance of i° 21' 45", with an annual diminution 

 of 19", and it may be observed either at its culmination or elongation, or 

 at any other known time. The time selected for the observation must 

 depend on the circumstances of the observer. If he has not the means 

 of determining his local time within fifteen seconds, he must take the star 

 at its elongation ; but if he can know the time, he may observe whenever 

 it is most convenient to himself. The culminations offer the advantage 

 of giving the meridian at once, without computation or correction for 

 azimuth ; but neither at the culmination nor at the elongation can double 

 observations be taken to eliminate any error in the adjustment of instru- 

 ments, and if the single observation be missed at the moment, it cannot 

 be repeated till the next night. It may therefore be more convenient to ob- 

 serve without reference to these ; but in that case the local time must be 

 known, the azimuth of the star computed, and the proper correction applied. 



These observations may be made with a theodolite or transit, or, for 

 want of these, we may use simply a plumb-line, with a compass-sight, or 

 anything with a small hole in it to look through. Any heavy body sus- 

 pended by a string will serve for a plumb, and it may be suspended in 

 water to give it greater stability. It should not, however, even then be 

 used in any considerable wind, as this will cause it to deviate from a 

 vertical. South of the plumb-line, and at a convenient distance, fix a 

 board firmly in a horizontal position, upon which a small piece of board, 

 with a compass-sight affixed, may be moved east and west. Bring this in 

 line with the star and plumb-line, and follow the star until the time of 

 culmination, then fasten the compass-sight, and the meridian is secured. 

 Or, if the time of an elongation be selected for the observation, bring the 

 compass-sight into line a little before the time, and follow the star till it 

 begins its return. This line, with the proper correction for azimuth, will 

 be the true meridian. So, also, if the observation be made at any time 



