30 Land Magnetic Observations, 1905-10 



Constants of Dover Dip Circle No. 205 Concluded. 



Formula: 



Ci = K/m = H sec / sin u sec /', 



Cd = Kim = H sec / sin u\, 



C = H sec /V sin u sin ui sec /'. 



Where / = magnetic dip, I' = loaded dip, u = I I',U\ = deflection angle, // = 

 magnetic horizontal intensity; log Ci at t = log Ci at t (/, t)q; log Cd at i = 

 log Cd at t, + {lt t)q where 1, is the temperature for which constants are given 

 and q is the effect of one degree difference in temperature on logarithm of constant. 

 The usual value of q is of the order 0.00010 but no determinations have as yet been 

 made for these needles. (It is much preferable and requires very little extra time to 

 observe invariably both loaded dip and deflections as log C is very nearly constant 

 and requires no temperature correction. Log C is furthermore free from eff^ect due to 

 change with time in the magnetic moment of the deflecting needle.) 



SPECIMENS OF OBSERVATIONS AND OF COMPUTATIONS. 



To assure uniformity in methods of observation and of computation both in 

 the field and in the Office, printed forms are used throughout the work. These are 

 shown by the specim.ens contained in this section. With the aid of the explanations 

 given in the three preceding sections, the reader will doubtless have no difficulty 

 following the forms. 



First are given (pages 31-37) the magnetic and astronomical observations, 

 together with the computations, for a typical station in medium magnetic latitude, 

 viz., Tekrit, Russia. Pages 31-33 contain the astronomical observations for the 

 determination of latitude, azimuth, and time (longitude), and the necessary com- 

 putations. The observations and computations required for the determination of 

 the three magnetic elements, the horizontal intensity, inclination, and declination, 

 as obtained with a magnetometer and a dip circle, are shown on pages 34-37. In 

 the bottom table, page 37, is given a general summary of the results obtained at 

 this station. 



Specimen total intensity observations made at Middlebury, Vermont, with a 

 dip circle, will be found on pages 38 and 39, as also the computations, and on page 

 40 are given specimen declination observations for the same station, using the 

 compass attachment belonging to the dip circle. 



Finally, on page 41, are shown inclination observations made with an earth 

 inductor at Cheltenham, Maryland. 



