190 
PROCEEDINGS OE SECTION A. 
The value for Alexandria-Suez, deduced from the two above, 
weighted in terms of their respective mean error, is — 
h. m. sec. 
Interval Alexandria-Suez — 0 10 39T20 db 0*073 ... III. 
(7c) Suez-Aden.— Observers • Dr- LBw at Suez, Dr. Gill at Aden. 
Time observations at Aden were made with some difficulty ; in fact, 
“ opportunities for observing were few and unsatisfactory” — (9) page 5. 
At Aden, the distance between the observing station and the telegraph 
office where signals were sent and received was nearly two miles. 
The operations were very limited, and the result depends on time 
observations of the single night of 31st January, and on the exchange 
of galvanic signals on the two nights of 30th and 31st January. This 
result was computed by Dr. Copeland — (9) page 196-227. 
(7c,) Suez- Aden . — Same operations as in (7c). Result given by 
Dr. Auwers — (7) page 61. 
(7c TI ) Suez- A den. — Observers: Captain (now Colonel) Campbell, 
R.E., at Suez; Captain (now Colonel) Heaviside, R.E., at Aden. 
Station at Suez the same as Mr. Hunter’s. Station at Aden a few 
yards north of the cable offices at Telegraph Bay. These officers 
aimed at the highest refinement possible, and had at their disposal the 
necessary equipment and conveniences wherewith to attain their 
purpose — (11), Part I., Chapter I. Their transit instruments were of 
similar dimensions and workmanship (5" object glass, with collimators, 
and means of levelling by mercury reflection, <fcc.) They recorded 
observations by chronograph. Galvanic signals were always exchanged 
directly between the stations, being sent by hand, and simultaneously 
recorded on both chronographs. Their operations were repeated on 
the six nights of 1877, May 25, 26, 27, 28, 29, and 30, giving very 
accordant results. Their personal equation was determined on four 
nights in April, 1877 ; and although it was not redetermined after the 
expedition, no serious consequences may he feared on that account. 
The observers themselves are confident that it remained fairly 
constant — (11), Part L, page 34. On the other hand, if their usual 
mode of observing was liable to sudden changes of considerable 
magnitude (of which there is no evidence), a redetermination after the 
expedition would have given very little help in finding the actual 
changes that took place at Suez and Aden. The only disadvantage in 
this measurement is to be attributed to the unsteadiness of the station 
at Suez, as already pointed out in (li) — (11), Part II. 
(1) Difference of longitude between Dr. Gill’s and Captain 
Heaviside’s station at Aden. 
This was determined by a careful triangulation, made under the 
direction of Captain Heaviside — (11) App., Part II. 
We have, then, for the interval Suez- Aden reduced to Mr. 
Hunter’s station at Suez, and Captain Heaviside’s at Aden 
h. m. sec. sec. 
4 { /*) + (i x ) } - / 0 49 42*839 + 0*120 
(Jc it ) 0 49 42*662 ± 0*060 
The difference between these two independent results is 0*17 /sec. 
Combining them according to their mean errors, we liave 
h. m. sec. sec. 
Suez- Aden ... 0 49 42*697 i 0*054 ... ... TV. 
