MAGNETICAL INSTRUCTIONS. 17 



I being the Intensity at the primary station, and I' the intensity derived by the method of constant 

 weights at the other station. 



The values of w' thus computed for all the stations where the weights and deflectors were both 

 used, being projected in a graphical representation with the corresponding values of v, the former as 

 ordinates the latter as abscissae, a line drawn by the eye through the terminations of the ordinates will 

 give the values of w for each degree of t/ produced by the deflector.* 



A non-continuity in the line freely drawn through the terminations of the ordinates, would point 

 to a diminished force in the deflector ; in this case, a second table for the value of w" corresponding to 

 the period of the weakened force is requisite. 



XOTE. Mr Fox prefers using the weights alone for determining the intensity, as when the 

 deflectors are used, it becomes necessary ii comparing observations made at different epochs to ascer- 

 tain not only the magnetic state of the needle itself at these epochs, but also that of the deflectors, as 

 any change in either the needle or the deflectors will render the observations non-comparable. 



It must be observed that if the needle or deflectors have lost magnetism the angle of deflection 

 will be less when the deflectors are used, and greater when weights alone are used ; whereas if the 

 force of the earth's magnetism only should be diminished by a change of station, the angles will be 

 increased by both methods. Nevertheless, under some circumstances the method by deflectors is 

 preferable to the method by weights, as it avoids the risk of injury consequent on the exposure of the 

 needle. This is particularly the case at sea ; in bad weather more especially. 



Temperature Correction, 



Needles. In the case of most needles their magnetic state is slightly influenced by the tem- 

 perature becoming weaker with increasing and stronger with decreasing temperatures. It is necessary 

 therefore to reduce the several results obtained with any particular needle to what they would have 

 been had all the observations been made at an uniform temperature. This is done by applying a cor- 

 rection to the several values of I' calculated as before described. The correction (which is additive 

 with increasing temperature when weights are used) is computed by the formula 



temp : correction = j I' (f f) 



in which t is the temperature at the base station, t the temperature at the station to which I' cor- 

 responds, and q a coefficient for 1 of Fahrenheit. 



q has occasionally been experimentally determined for each needle by observations in high and 

 low temperatures at a magnetic observatory, where means existed for eliminating the actual changes of 

 the magnetic force taking place during the time necessarily occupied by the experiments. 



The temperature correction can also be obtained by placing the instrument under a glass receiver, 

 and admitting heated air under it. The needle being deflected at a given angle from the Dip by a 

 given weight, the increase in this angle, corresponding to an increase of temperature, being noted. 

 Then 



- 



(t-f) 



* See" Contributions to Terrestrial Magnetism," No. III., PhiL Trans, for 1842, for a full account of this method of 

 forming a table of equivalent weights. 



VOL. II. C 



