ROYAL IRISH ACADEMY. 
43 
Tables of these results are given, from which he at first hoped to 
ascertain the law connecting the power with these variables. The problem 
appears too complicated to be solved by experiment alone, but he offers 
them as useful data both for theoretic research and practical application. 
Various magnets are compared: in general, the lifting power is 
greater the shorter the magnet, and the closer the spires are brought to 
its poles; if it be intended to act at a distance, or to magnetize hard 
steel, it should be long, and uniformly covered with spires. 
Beyond 2 1 diameter, or even less, the central part of the magnet 
seems not to contribute to its power. 
The power decreases with a rise of temperature if the magnet be 
iron, the rate varying with its length and section; if of hard steel, it 
increases, and much more rapidly. 
Both with iron and steel there remains magnetism if the exciting 
current be withdrawn; in the latter case it is permanent and of large 
amount, and is not destroyed by reversing the current (unless that be 
of a certain power), even though it produce a temporary reversal of po¬ 
larity while passing. The power of steel is with ordinary exciting 
forces far less than that of iron, but with higher they tend to equality. 
If a table of the successive powers of a magnet and the correspond¬ 
ing exciting forces be examined, it is seen that they are not proportional, 
except approximately at the beginning of the series. The increase of 
the first for a given increase of the second diminishes constantly, and 
so as to show that in every instance there is a maximum which no 
amount of current force or number of spires can pass. The precise re¬ 
lation between the power and exciting force has not yet been determined, 
but he finds that the following empirical formula represents very well, 
except for the very lowest powers, the action of the seven magnets with 
which he worked— 
L = 
Ayfr 
in which A is the maximum power; the exciting force measured by 
the quantity of the current x number of spires; and B a constant 
which may be called the modulus of the magnet, and seems to possess 
some remarkable properties. It is the yjr which produces a power = -JA, 
and below which the permanent magnetism is not reversed completely. 
Below it, also, what he calls residual excitation, that which remains 
after the current ceases, and till the keeper is raised, varies; above it, 
it is constant. 
The paper concludes with a summary of its contents and those of 
the two preceding it. 
A perfect copy of Charles Brooking’s map of Dublin, published in 
1728, with a view of the city, and fronts of the public buildings, was 
presented by Miss Wilkinson. 
A list of donations of books presented was read, and thanks voted 
to the donors. 
