236 Mr. Michael Donovan on certain Improvements in 



horizontal portion is attainable by means of a detached spirit level and levelling 

 screws ; and the balance of the needle is regulated by a very small slider made 

 of brass foil. It will be often found necessary to have a slider even on each 

 member of the compormd needle ; for as the intensity of its magnetism is some- 

 times stronger, sometimes weaker, in consequence of spontaneous or induced 

 changes, the dip will affect it more or less; and hence the necessity of the 

 sliding equipoise to maintain the horizontal position in so narrow a chamber. 

 The second slider becomes necessary when the poles are reversed in the man- 

 ner hereafter described. 



The level and levelling screws afford the means not only of rendering the 

 plane of the chamber perfectly horizontal, but of giving true verticity to the 

 pillar from the cross-bar of which the needle hangs. Without such a precaution 

 the spindle of the needle would not freely rotate in the small hole made for it 

 in the meridian line of the circular plate, or rather between the two semicircu- 

 lar plates. 



To give greater precision to the centrality of this spindle, the hole in its 

 top is made exceedingly small, and through it is looped the suspending silk 

 fibre, the other end of which is looped through an equally small liole in a pin 

 which may be secured by a thumb-screw, in the cleft end of a horizontal cross- 

 bar, moveable in every direction at the top of the pillar. The use of having 

 the holes so small is to secure the silk from shifting, and thus altering the ba- 

 lance or position of the needle. The length of silk fibre which I conceive to 

 be sufficient is 7^ inches, exclusive of what is looped above and below. 



When occasion requires the compound needle to be removed from the gal- 

 vanometer, the thumb-screw which screws against the pin in the cleft of the 

 cross-bar at the top of the pillar is to be loosed ; the pin with the silk fibre is 

 to be drawn out ; the two semicircular plates which lie within the graduated 

 circle are to be pushed up from below, and the needle withdrawn. This faci- 

 lity of removing and replacing the needle is a great advantage when its mag- 

 netism requires regulation ; and beside this the silk fibre often becomes twisted 

 by the rapid spinning of the needle which certain experiments occasion and even 

 require: it is prevented from untwisting by the polarity of the needle; but if 

 the needle be held between the fingers while the pin hangs down by its fibre of 

 silk, the latter will imtwist itself; and when the pin ceases to spin, the whole 



