331 



and by a window at each end. It contains six stone pillars 

 for the support of the instruments : these are imbedded in 

 solid masonry beneath the floor, and the supports of the 

 floor are framed around them, so that they are completely 

 insulated. 



The elements on which the determination of the earth's 

 magnetic force is usually based are, the declination, the in- 

 clination, and the intensity. If a vertical plane be conceived 

 to pass through the direction of the force, that direction 

 will be determined when its inclination to the horizon is 

 given, as well as the angle which the plane itself forms with 

 the meridian ; and if, in addition to these quantities, we like- 

 wise know the number which expresses the ratio of the in- 

 tensity of the force to some established unit, it is manifest 

 that the. force is completely determined. 



For many purposes, however, and especially in the deli- 

 cate researches connected with the variations of the magnetic 

 force, a different system of elements is preferable. If the 

 intensity be resolved into two portions in the plane of the 

 magnetic meridian, one of them horizontal and the other 

 vertical, it is manifest that these two components may be 

 substituted for the total intensity and the inclination ; while, 

 at the same time, their changes may be determined with far 

 greater precision. The former variables are connected with 

 the latter by the relations 



xztrcosw, Yzritsinw; 

 in which R denotes the intensity, x and y its horizontal 

 and vertical components, and u the inclination. It will be 

 easily seen that the variations of u and r are expressed in 

 terms of the variations of x and y by the following formulas : 



bu — sin u cos ul ) ; 



\Y X/' 



, • 2 SY 



4- sin w — . 



Y 



Mr. Lloyd then proceeded to describe the instruments 



— = cos u — + sin u — . 



R X Y 



