INTRODUCTION. 
XIII 
ticularly the case when magnetic bearings are made use of in the 
construction of a chart, a mode of surveying which, of course, 
will only be resorted to when absolutely necessary. In such cases, 
it will be proper to observe the variation of the needle upon the 
same course as that on which the bearings are taken ; by this means 
a degree of correctness may be attained, which would be little 
expected by those who are unaccustomed to adopt this precaution, 
and most of those errors avoided, which it has been usual to attri- 
bute to a defect in the compasses. 
To avoid unnecessary repetition in the course of the following 
Narrative, it must be remarked that all the bearings are the true ones, 
unless otherwise expressly noticed ; and the whole of the latitudes are 
Norths and the longitudes West from the meridian of Greenwich. The 
temperatures were registered entirely by Fahrenheit’s thermometer, 
and it may be necessary to inform the general reader, that the 
signs + and — preceding any number of degrees, signify above or 
below zero of that scale. i 
The temperature of the sea at different depths was obtained, unless 
otherwise noticed, by Sixe’s self-registering thermometer, confined 
in an iron case, and attached to the deep-sea lead. The bottle used 
for bringing up water from different depths below the surface, was 
invented by Doctor Marcet, expressly for the use of this Expedition. 
It consists of a strong and heavy cylindrical box of cast iron, having 
a small aperture at each end ; through these apertures passes a bolt 
which, when let down into its place, completely closes them, but 
when held up by means of a catch in the upper part of the box, 
allows the water to pass through them freely, both at the top and 
bottom. Being thus set, it is let down to any depth required, by 
