778 
THE YOYAGE OF H.M.S. CHALLENGER. 
During the stay at Tahiti a careful 
examination was made of the coral reefs, 
and a survey was specially conducted to 
ascertain the seaward slope of the barrier, by 
Lieutenant H. Swire, who writes as follows : — 
“ I gladly entered into co-operation with 
Mr. Murray, under whose guidance and 
general superintendance the examination was 
conducted, and by his desire made a survey 
of the seaward face of the reef, carrying the 
soundings from the water’s edge to a depth 
of 1 80 fathoms by boat, the lines of soundings 
being afterwards extended by the ship when 
under steam, to a depth of 680 fathoms 
(see fig. 269). 
“ By sounding to seaward on lines 200 
yards apart, and perpendicular to the direc- 
tion of the reef, taking the depth at equal 
intervals and carefully fixing each position 
by angles ; and in addition, by fixing the 
position on each line of certain given depths 
— as for instance 5, 10, 15, 20, &c., fathoms — 
it was considered that a good insight into 
the configuration of the reef would be 
obtained, whilst by ‘ arming ' the sounding 
lead with the usual mixture of white lead 
and tallow in order to ascertain the nature 
of the bottom at each cast, and by making 
free use of drag nets, a dredge fitted with 
tangles, a water telescope, &c., the investiga- 
tion into the other conditions of the reef 
could be simultaneously carried on. 
“The portion of the reef selected was 
1120 yards in length, the greater part being 
in general direction a straight line, which cir- 
cumstance somewhat simplified the matter, 
as a single base line sufficed for the greater 
portion of the reef. 
“ In order to obtain, soundings at given 
