Phenomena of Polarized Light. 89 
trivances which will be described at some future time, I have 
been enabled to obviate mostly the principal difficulty, viz,,-the 
time of the coil, or rather to turn it to a practical account, and to 
this in part is due my recent great success in the application of 
this power. I have now an engine of nine to ten horse power, | 
the details of which cannot be given here. 
I notice that Mr. Hunt in a recent address before the Society 
of Arts, in London, describes the effect of secondary currents and 
also the effect of motion in diminishing the power of electro-mag- 
netic engines, and claims that the subject was then for the first 
time specifically brought forward. In 1838 I prepared a special 
paper upon this subject, which was published in Silliman’s Jour- 
nal at that time, in which I fully set forth and anticipated al 
these difficulties, and also shewed by an instrument termed the 
Magneto-Electric Multiplier, how the secondary current might be 
used to increase or diminish the motion of a revolving magnet. 
The method of determining the time for the rise of the cur 
rent in my coils by means of the metronome, and the size of the 
sparks is not to be relied upon for entire accuracy, and I have in 
“one ee an instrument by which it can be done more per 
ectly. . 
| 
Note.—In a recent letter from Dr. Page, he mentions the fol- 
lowing experiment with his powerful magnet.—Eps. 
“Ihave just completed a grand experiment with a huge iron 
bar and helix, with the following results. The bar weighing 
532 pounds placed within the helix, is made to start up in the 
coil and vibrate in the air without visible support. It requires 4 
force of 508 pounds additional to its own weight to pull it out of 
the helix, so that it is equivalent to lifting a bar in the helix, of 
1040 pounds in weight. After this it would seem quite easy to 
Sustain masses of iron weighing many tons. The full time re- 
quired to charge this magnet and raise the galvanic current to its 
‘Maximum is two seconds. Nine-tenths of the charge 1s attained 
iN one second.” 
*~; 
Arr. X.—A new figure in Mica and other Phenomena of Polar- 
ized Light; by Prof. Cuas. G. Pace, M. D., Washington, D. C. 
In order to exhibit well the system of rings in mica, we must 
select the clearest specimens of uniform appearance and density, 
and the piece should be at least ;';th of an inch thick. If the 
_ Mica be very clear it may be somewhat thicker to advantage. 
‘he inclination of the resudtant azes is so great in mica, that but 
xp Suures, Vol, XI, No. $1—Jan, 1851. : 
