108 REPORT—1849. 
terrestrial induction made to aggrandize the idio-polarity, to lift and sustain 
a common sewing-needle ; they had all two well-defined poles at the extremities’ 
and one neutral point, but the poles were seldom of quite the same intensity. 
The set No. 3, viz. those coated with coal-tar and exposed to abrasion of 
traffic only, when submitted to a similar examination, present quite the 
same characters; each bar was powerfully idio-polar, with two well-defined 
poles of slightly unequal intensity and one neutral point, All the pheno- 
mena of terrestrial induction could of course be made evident with these as 
well as the other sets. 
What is very remarkable, the intensity of magnetic polarity was quite as 
great in those bars and in the set No. 2 after removal from the Dalkey line, 
where they were travelled over in both directions, as after removal from the 
Kingstown, where travelled over only in one. In the latter case however the 
direction of the poles of each bar, as it lay in the line of railway, was that 
due to terrestrial induction, 7. e. the south pole of the bar faced the north 
pole of the earth and vice versd ; the opposite was the case with the bars on 
the Kingstown line of set No. 2, as already detailed. 
These facts are sufficient wholly to overturn the views suggested by Mr. 
Nasmyth, in his letter already given, as to the peculiar effects of traffic in 
one or in both directions. 
In connexion with this subject, I have also examined the magnetic con- 
dition of old wheels and axles that have run for a considerable time on rail- 
ways. In every case these ran more or less in both directions; the results 
presented are very perplexing and variable, but in general I find the axle is 
more or less idio-polar. The wheel also is very often feebly idio-polar, the 
poles being at the nave and periphery ; but when a pair of wheels and their own 
axle stand on the rails in the usual position, the axle being idio-polar, then 
by induction from the axle and by terrestrial induction conspiring, a tolerably 
distinct polarity of the whole wheel jg produced, the nave presenting a pole 
opposite to that of its own end of the axle, and a pole opposite to this being 
found opposite the extremity Fig. 10. 
of every spoke round the 
wheel-tire orperiphery: thus 
when the wheel is rolled 
along the rails where the lat- 
ter have been long in use and 
are themselves polar, the in- 
tensity varies with the posi- 
tion of the wheel overa given 
length of rail, and may be a 
maximum when the wheel 
rests over a joint beween two 
rails, viz. over the polar extre- 
mities of the contiguous rails, 
I endeavoured to ascertain whether the total intensity of the six bars censti- 
tuting the set exposed to abrasion only, or of the six bars exposed to abrasion 
and corrosion, was the greater, but not having a suitably mounted mag- 
netometer, I was unable to satisfy myself fully; so far as my trials went, 
however, there seemed but little difference appreciable in either case, and 
that equally so whether the two different sets were rolled over both ways, or 
only in one direction. ; 
I ascertained also that the polarity of each bar, as it lies in the line of 
railway, is somewhat increased in intensity by induction from the bar lying 
parallel to it in the opposite side of the railway line. This effect however 


