34 EESEARCHES ON II. 



upper part of the stem AD \s square, and the lower is a screw, so that by turning 

 the nut C the table tt' and the globe or circle 00' are raised without any angular 

 motion.^ On this table tt' is a board m'm" sliding in a dove-tail channel, by 

 means of which the globe is moved eastward or westward; and a similar piece hh' 

 movable in the same way on the board m'm" permits motions towards south and 

 north ; the stand being conveniently fixed relatively to the magnetic meridian, all 

 these motions are made with great facility. The positions of the globe are deter- 

 mined by scales divided into tenths of the radius, applied to every sliding piece, 

 and to the square part of the stem. Every sliding part has also pressure screws, 

 as y, v'. The circle or globe is steadily fixed to two arms hh', h'h" placed at 

 right angles to each other. It is placed in the vertical plane by a plummet, and 

 the middle line of the wires made to coincide with the magnetic meridian by 

 using a magnetic needle 14 inches long. 



V. The contrivance for carrying the small magnetic needle, which is destined to 

 measure the intensity of the currents, is represented in Fig. V. A board BB' is 

 attached to the wall, and on it a sliding piece E, E' carries two hinges, around which 

 the piece Qupq is movable. Half a circle cc' against which a screw S presses, keeps 

 it at any required angle. The board EE' which carries this part of the apparatus, 

 is movable up and down in a dove-tail channel on the other board BB', and can 

 be fixed to any height by the screw W. A strong brass bar qo, about 8 inches 

 long, is attached to the arm pq, and supports near its extremity o a circular plate 

 of copper 3 inches in diameter, and all the suspending apparatus for the needle. 

 This consists of the said circular copper plate, on which the graduations for mea- 

 suring the deviations of the needle are marked. This plate supports a glass 

 cylinder g to protect the needle from the agitation of the air. Outside the glass a 

 strong copper rod carries a mirror II' and sends out a small arm a, to which the silk 

 fibre suspending the needle n is attached. The use of this mirror is to throw light 

 on the circular divisions when the apparatus is placed opposite to a window, and 

 also to give a reflected image of the circle which is to be observed with a telescope 

 (Fig. VI). For this last reason the cylinder g is quite open at the top. By using 

 the telescope we have the advantage of obtaining a magnified image of the circle, 

 and therefore a more exact appreciation of the slightest variations of position in 

 the needle ; besides this, the errors of parallax in the reading of degrees are com- 

 pletely avoided. The reading itself is rendered very easy, which in some cases 

 would be almost impossible, being prevented by the material body of the globe. 

 But there is also a more important advantage, which is that the observer may be 

 far from the globe, and look at the needle through the telescope while he turns the 

 rheostat with his hand. In this way time and labor are saved, and the observa- 

 tions are more accurate. 



Plate III., as we have said, represents all this apparatus together ; the course of 

 the current is indicated by the arrows. From the copper plate G it passes to the 



• In the figure we have delineated only one circle, because the entire glohe would have hidden the 

 principal part of the figure. 



