the Intensity of the Earth's Magnetic Force. 545 



when employed as a deflector, should be protected from the heat of the ob- 

 server's body by a small case of glass or of metal. 



The method here proposed appears to offer the following advantages to the 

 travelling observer : — 



1. It is applicable, with equal accuracy, at all parts of the globe. 



2. It dispenses with the employment of a separate instrument for the deter- 

 mination of the magnetic intensity, and with the separate adjustments required 

 in erecting it. 



3. The constants to be determined — the magnitude of the added weight, 

 and the radius of the pulley by which it acts — can be ascertained with more 

 ease and certainty than those which are required in the method of vibrations, 

 and are less liable to subsequent change. 



4. The observations themselves are less varied in character than the usual 

 ones, and may be completed in a shorter time. 



