cfhot Iron between the white and hlood-red Heat. 347 



action of the iron decreases in the like cases, and (passing 

 tlirough zero in the plane of no attraction) has its quantity of 

 attraction different when placed towards the upper or lower 

 extremity of the bar. 



The negative attraction has also the same change of cha- 

 racter in the upper and lower extremity of the bar ; but as it 

 increases towards the middle, it appeared to pass through a 

 maximum to arrive at that change, Avhich seemed wholly in- 

 explicable ; and I must acknowledge that, after all the experi- 

 ments I have made, it still remains so. It is at all events cer- 

 tain, that the least change of position of the compass when 

 near the centre of the bar, changes altogether the quantity and 

 quality oi" this negative action. 



In the experiments detailed in the following table, I used 

 four different bars, each 25 inches long, and 1 J inch square ; 

 two of them of cast iron, denoted in the first column by C. B, 

 No. 1 ; C. B, No. 2 ; and two of malleable iron, denoted by 

 M. B, No. 1, and M. B, No. 2. 



I had also two other bars, one of cast and one of malleable 

 iron, of the same dimensions, v.hich were not heated, but kept 

 as standards for determining the quantity of cold attraction, 

 as this could not safely be done by the bars used in the expei'i- 

 ments after being so repeatedly heated. 



The time employed in each experiment was about a quar- 

 ter of an hour : the white heat commonly remained about three 

 minutes, when the negative attraction commenced; this lasted 

 about two minutes more, when the usual attraclion took place: 

 this sometimes arrived at its maximum very rajiidly, but at 

 others it proceeded increasing very gradually ; and connnonly 

 within the time stated above, the needle had been found per- 

 fectly stationary. 



In the table, to avoid confusion, that attraction whicli took 

 place according to the known laws of cold iron is marked ^j/?«, 

 whichever end of the needle approached the iron, and the 

 opposite attraction is marked ininua. For example: when the 

 compass is above the centre of tlie bar, the north end of the 

 needle should be drawn towards tlie iron ; but when the coni- 

 j)ass is below the centre, the south end should ajiproadi tlie 

 iron ; these therefore are both marked plus, and the ctuitrary 

 attraction at the red heat is marked miims. 



X X 2 T.MJLE 



