E. Loomis on the Variation of the Magnetic Needle. 167 
cable without endangering it, and consequently a high speed in 
transmission obtained. For a first trial on the Atlantic cable I 
would use a Smee battery of about 500 pairs of plates, each 
having 20 square feet of surface; yet it may be found on tri 
that plates of much larger size may be required. 
Although the present cable may be incapable of working with 
a satisfactory degree of certainty and rapidity, even with the 
_ best electrical management, yet this would not demonstrate that 
there cannot be a satisfactory electrical communication across 
the Atlantic. Certainly I will be justified in saying that the 
present state of the engineering art and the experience already 
obtained in laying submarine conductors, can lay down a new 
cable capable of transmitting 100 letters per minute, if it is con- 
structed and worked in full regard to the principles of electricity. 
Arr. XVIII.—On the Variation of the Magnetic Needle at Hudson, 
Ohio; by Eras Loomis, Professor of Mathematics and Natu- 
ral Philosophy in the University of the City of New York. 
pass by Gam : 
long, supported by fibres of untwisted silk, and resting in a 
stirrup which admits of easy reversal. It is now proposed to 
annual change of the variation. 
le at different hours of the day, it is necessary to apply a cor- 
rection to reduce each result to the mean variation of the month 
