386 
GREENLAND VOYAGE. 
The only objection to the multiplication of 
light-houses, (excepting the expence, towards 
which there is no one who knows and experiences 
the value of them, that would not cheerfully con¬ 
tribute), is the possibility of mistaking one light 
for another. There are several principles, how¬ 
ever, now usually employed as a distinction of 
lights, that are quite effectual when known. A 
fixed, or uniform light, and a revolving light, are 
differences so striking that they cannot be mis¬ 
taken : these are also varied by the use of coloured 
skreens; and the latter are further defined by a dif¬ 
ference in the period of the revolution of the lights. 
Some other principles might be adopted, I conceive, 
that would render the lights still more effective. 
In all elevated lights, for instance, a simple indi¬ 
cation of direction and distance might possibly be 
given, by the use of coloured skreens, to be applied 
only in such a way as to produce their effect a- 
round the limits of any particular danger. Or 
the same object might, perhaps, be accomplished, 
by having a secondary light in the same building, 
that should be so guarded by opaque skreens, that 
it could only be seen by the navigator on his ad¬ 
vance into danger ; or what would amount to the 
same thing, it might be eclipsed in certain posi¬ 
tions by an opaque skreen, so as to mark by its 
disappearance the proximity of danger. And in 
