368 On the lest Kind of Steel and Form 
Experiment 20. 
Softened from the middle to a greyish white; . Directive 
ends hard. force. 
Parallelogram PM Pa ee ee ie oe 
Rhombus eee ew) ale Tite StS akon 
Pierced Thomuus: “5s 0. (Si ee 
The tempering was carried throughout the needles, the paral- 
lelogram was reserved for another purpose. 
Experiment 21, 
Softened throughout to a greyish white. . Directive force. 
Rhombus EPR R RIA PRPS: 
BICRPED THPMDUS is ee ne isn, pe ae 
Experiment 22. 
Softened throughout to a greyish white, the Directive 
ends hardened at a red-heat force. 
Rhombus co Soe as ee SSS ty SR 
Piereed:rhombus, -s.. -<..°-+..° 7 2ePeSs 
Experiment 23. 
Hardened throughout, and then softened to a Directive 
greyish white, as in Experiment 21]. force. 
POMS: ..4) ow f,0 \5.0.07-.ben towel aw setae 
Pierced rhombus... ...- 62 se, «+ 1180 
This last series of experiments presents a curious circum- 
stance. From the experiments made by Coulomb, as well as 
from the general tenor of my own, the rhombus is found capable 
of receiving a greater directive energy than the parallelogram; 
yet here we perceive that the parallelogram, though formed of 
the very same plate of steel as the other needles, is not only un- 
der every circumstance superior to the rhombus, but also to the 
pierced rhombus. It is difficult to form any plausible conjecture 
as to the cause of this difference. 
The weight of the rhombus in Experiment 10, made of cleck- 
spring, was 63 grains; that made of saw- blade weighed 120 
grains, or very nearly double. The directive energy of the for- 
mer, after having suffered great deterioration, and when not 
tempered in the most favourable manner, compared with the 
greatest directive energy of the latter, was as 600 to 1210; but 
if we refer to Experiment 6, it may be seen that the greatest 
directive energy of the clock-spring rhombus was S44, which 
gives it an advantage of about one-third in directive energy over 
a needle of equal weight made of saw-blade. 
From Experimeut 20, it should seem that a needle is suscep- 
tible of the greatest directive power, other circumstances being 
similar, when it is hardened throughout at a red-heat, and theu 
softened from the middle to within an inch of the extremities, 
till the blue celour which arises has again disappeared. 
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