made in the University of Utrecht. J 71 
to which succeeds a stronger one to the west. A bar of zinc is 
now introduced in the fluid, and successively made to touch the 
ends A and B. When this zinc touches the end A, the devia- 
tion increases to the east, but if it touches the end B, the devia- 
tion increases to the west. Both ends being joined by the zinc, 
the deviation is to the west. 
Instead of a bar of zinc, another of copper is introduced, and 
successively made to touch the ends of the zinc. If it touches 
the end A the deviation is to the west, if it touches the end B 
the deviation is to the east ; if both ends are joined by the cop- 
per, the needle wheels about its axis, turning first through the 
west. , 
14. The apparatus was then so far altered as to connect both 
ends of the zinc by a bar of copper. The disposition was as 
follows : — The ends A and B were united by a bar of copper, 
C extending between them, as shewn by dotted lines. This 
part of the apparatus, as in the preceding, was turned to the 
south, and dipped in water. A thermometer proved the tem- 
perature of this water about 56°. When sulphuric acid was 
added, the needle immediately deviated strongly to the east ; 
and, as the temperature augmented to 92°, the deviation be- 
came strongly westerly. 
15. Instead of zinc, iron was now made use of, and a hoop of 
3 or 4 feet long, 2 inches wide, and ^th of an inch thick, was 
bent, as shewn in Fig. 14. 
The plane of this bent hoop was placed in the magnetic meridi- 
an, and two needles, one in C, and another in D, were so ar- 
ranged as to have their directions in the same plane. Sulphuric 
acid was then poured on the water in the cup, and instantane* 
ously the point of the needle turned 70° to the east. Soon af- 
ter, this deviation decreased, and became as strongly westerly , 
and remained so for several minutes. It then slowly returned 
to the magnetic meridian. When returned in this situation, ni- 
tric add was added to the sulphuric acid water already in the 
cup. By these means the deviation became 90° to the east, and 
remained so for a considerable time. Then a lump of zinc was 
thrown in the acids, and the easterly deviation changed to a 
westerly. After all this a piece of copper was introduced, which 
made the needle turn on its axis. While the needle placed in C 
was thus deviating, the others in D followed in a contrary di- 
