OF MAGNETIC INSTRUMENTS. 



173 



FIG. 2020. 



FIG. 2019. 



2015. Horizontal Steel Bar Needles, for delicate magnetic investigations. These 



needles are of various shapes and dimensions, mounted with central agate or 

 ruby caps, and every precaution taken in selecting the finest steel as well as 

 its treatment in forming the needles, and the method employed in the 

 magnetization; 2-inch., 2s. 6d. ; 3-inch., 4s. 6d. ; 4-inch., 6s. ; 6-inch. 076 



2016. BBASS STAND, with fine steel point for the suspension of horizontal magnetic 



needles (fig. 2016), p. 175 . . . . 2 6 to 4 6 



2017. POUILLET'S ASTATIC NEEDLES, composed of a pair of steel needles alike in their 



form and intensity, placed parallel one above the other on a common centre of 

 motion with the similar magnetic poles in opposite directions, by which the 

 directive tendency of the earth's magnetism is nearly neutralized if not over- 

 come (fig. 2017), p. 175 . * , 7 6 to 1 1 



2018. SMALL DIPPING NEEDLE, with slender brass graduated quadrant, upon which 



the needle shows the dip. If this be moved along a bar magnet, it illustrates 

 the relative situations and tendencies of a needle when acted upon by the 

 earth's magnetism. For when placed on the middle or equatorial part, the 

 mutual actions of the north and south poles balance each other, and cause the 

 needle to stand exactly parallel to the bar ; but as the needle is slid towards 

 either extremity, it will be inclined according to its distance from the magnetic 

 poles (fig 2018), p. 175 . . . 18 to 1 10 



2019. Small Dipping Needle, with graduated circular brass ring, on which the 



needle shows the inclination or dip due to terrestrial influence . When the 

 apparatus is passed over a bar magnet, a popular illustration is afforded of the 

 action of the earth's magnetism (fig. 2019) . . 18 to 1 1 



2020. MAGNETIC NEEDLE, arranged to admit of its moving in a vertical as well as in a 



horizontal plane. This arrangement of the needle, with its standard and 

 graduated arc, furnishes an instrument adapted well to show the real influence 

 of terrestrial magnetism on magnetic bodies, having free motion in all 

 directions (fig. 2020) . . . . 1 1 to 1 5 



2022. MAGNETIC NEEDLES, mounted on stands, for ascertaining the polarity of 

 mineralogical specimens . . . . . 050 



