454 Prof. W. A. Jenkins on 



5. "Reduced" forms of the vapour-pressure equation are 

 obtained and an equation for the calculation of the critical 

 density of a substance. This gives consistent values, even in 

 cases such as that of water when the Mathias rule does not 

 hold. 



I wish to thank Professor Porter for his kindly criticism 

 and for the interest he has taken in this paper. 



XXXIX. On the Determination of u B: } By Walter A. 

 Jenkins, M.Sc, Professor of Physics, Dacca College*. 



IN the Philosophical Magazine of October 1913, a new 

 method of determining the Horizontal Component of 

 the Earth's Magnetic Field was described by the writer. 

 The method, suggested by Dr. Hicks, was the creation of an 

 artificial magnetic field exactly equal in intensity to twice 

 the Earth's field. Equality of the two fields was determined 

 by the equality of the times of swing of a suspended magnet 

 placed in both fields. It was there shown that the method 

 was capable of giving an accuracy of one part in 10,000 and 

 was as efficient a method as the Kew Magnetometer one. 

 But in both the Kew Magnetometer method and the Solenoid 

 method previously described, the chief part of the experi- 

 ment is the determination of a time of swing, and the limit 

 of accuracy of the methods is the accuracy with which the 

 time of swing can be determined. In the previous method 

 difficulty was experienced in obtaining a suspension fibre 

 sufficiently fine, strong, and short enough to allow the magnet 

 to oscillate for the period required for making accurate 

 observations and at the same time to conform to the rest of 

 the apparatus. Two methods will be described in the present 

 paper, both of which obviate this difficulty and make the 

 determination of "H' ) a short and reliable experiment. 



First Method. — The apparatus used is essentially the same 

 as that designed for the former experiment and the principle 

 of the method much the same. In this method, however, 

 the equality of the fields is determined by measuring the 

 angle of deflexion of the suspended magnet when under the 

 influence of both the Earth's field and the Solenoid field. 

 The Solenoid field is placed exactly at right angles to the 



* Communicated by the Author. 



