286 STAFF COMMANDER EVANS AND MR. A. SMITH ON THE MAGNETIC 
the steering-compass. In several iron-built ships purchased into the Royal Navy from 
ten to fifteen years after Mr. Airy’s observations, X averages at present about - 930. In 
the iron-plated ships of the present day it ranges from 'TOO to - 900. 
The following are its values in the iron-plated ships before mentioned. 
Warrior. 
Black 
Prince. 
Achilles. 
Defence. 
Resistance. 
Hector. 
Valiant. 
Royal Oak 
(wood- 
built). 
Standard compass 
•873 
•783 
•822 
•822 
•758 
•814 
•859 
•907 
Starboard steering 
•833 
•760 
•777 
•794 
•703 
•791 
•817 
•906 
Main deck 
•757 
•755 
•759 
•782 
•726 
•722 
•862 
The large value in the Warrior is evidently owing to the rifle tower, the small value 
in the Resistance, as compared to the value in the Defence, to the position of the com- 
passes with respect to the armour bulkheads as above described, and with reference to 
the armour-plating generally. 
Familiarity with the values of 2 and X in vessels of different classes, is of great import- 
ance in enabling us to deduce 95 and (§, by observations made without swinging. 
The mathematical theory from which the values of 2) and X are derived, supposes 
that the transient induced magnetism to which 2 and 1— X owe their values, is instan- 
taneously developed, and as instantaneously destroyed or altered as the ship assumes a 
new position. This we cannot suppose to be exactly true; but whether the time 
required for the soft iron to receive its new magnetic state as the ship swings is appre- 
ciable has been a matter of doubt. The opinion of the authors of the Report of the 
Liverpool Compass Committee (an opinion entitled to the greatest weight) was, that 
an appreciable time was required, and that the value of D in particular might be different 
according as the vessel was swung slowly or quickly ; we have not, however, been able 
to detect any difference in the values of D which can be attributed to any cause of this 
nature. 
The most remarkable feature, however, in X and 2 is the change which takes place 
with the lapse of time, indicating apparently a change in the molecular structure of the 
soft iron by which it becomes less susceptible of induced magnetism. This is shown 
clearly in the following Table : — 
