Decembee 6, 1895.] 



SCIENCE. 



759 



[1] Fi^X 



that on looking thi-ough the hole li, the 

 cross X is seen reflected in the mirror 

 of A with its intersection upon the ver- 

 tical line. The magnets are upon the back 

 of the mirror shown ^ natural size at Fig. 2. 

 D is a wing dipping into an oil cup and 

 acting as a damper. Of course, this method 

 of sighting brings the hole h and cross X 

 into the plane normal to the mirror. The 

 magnet M being placed in position, the 

 mirror is deflected and the sight P is slid 

 along the scale S until the cross is again 

 seen coincident with the line upon the 

 mirror. The difference between this and 

 the previous setting of the slide expressed 

 as a decimal of a meter will be the natural 

 tangent of the angle of deflection, and is 

 easily coii-ect to the third, with a fair ap- 

 proximation to the fourth place of decimals. 

 P being always set on the normal to the 

 mirror and its position measured upon the 

 tangent to a circle whose radius is one, the 

 reading is always the natural tangent of the 

 angle. This method of reading angles will 

 be found convenient in many other cases. 



VAEIOMETER. 



The method of reading angles above 

 described has been applied to an apparatus 

 for measuring the local variations in the 

 horizontal component of the Earth's mag- 

 netism with an error somewhat less than 

 1%. 



Fig. 4 shows the apparatus about one- 

 third natural size. It consists of a magne- 

 tometer M, similar to that shown in Fig. 

 1, so mounted as to be movable on a 

 slide A. B is a coil of copper wire of 

 about 5 ohms resistance whose support is 

 also movable upon a scale parallel to A, 

 so that M is upon the axis of B. Two 

 Daniel cells at C and C (C is removed in 

 the cut) send a constant current through 

 the commutator D and the coil B. The 

 sight P is similar to that shown in Fig 3. 

 The scale S is divided to read horizontal in- 

 tensity directly. That is with zero at the 

 center, distances d either way upon the 

 scale are computed accoi-ding to the for- 

 mula, d=^. H is the horizontal compo- 

 nent of the earth's magnetism, C the cur- 



