6
across the Pacific in an infected condition ; and it is necessary next to trace the
routes of ships from the endemic area to their destinations in Asia. The details
on this subject are given in Section III and only the following points need be
summarized here.
(1) At present all ships which leave America for the trans-Pacific voyage
to the East from San Francisco and more southerly ports use the Hawaiian
Islands as the first place of call. The conditions in these islands are throughout
the year favourable to the existence of yellow fever, but up to the present the
disease has been effectually excluded. The authorities fully appreciate the
danger of the introduction of yellow fever from Mexican ports, and very
thorough precautions are taken. After studying the local conditions and arrange-
ments in Honolulu I am of opinion that the port affords a strong protection
against the infection of Asia and the East Indies,
O
(2) On the usual route to Hong Kong the ships, after leaving Honolulu,
pass northwards into latitudes not as a rule favourable to the life of mosquitoes,
and the remaining ports of call are Yokohama and other Japanese ports and
Shanghai. There is a serious gap in our knowledge of the conditions in these
ports, for we do not yet know whether stegomyia fasciata occurs in them or not.
If it is present, and sufficiently abundant, the introduction of the yellow fever
virus might lead to an epidemic, but the climate is such that the disease would
entirely die out during the winter, and fresh importation would be necessary for
its reappearance,
(3) The climate of Hong Kong is more favourable to the existence and
spread of yellow fever, but again we are ignorant of the presence or absence
of stegomyia fasciata there. In all probability a thorough search during the
hot months would show that it is present, but quite possibly it is not very
abundant.
(4) The route from San Francisco via Honolulu to the Phillipines does not
necessarily take ships northward to Japan, but until San Francisco or Honolulu
becomes infected such a route is not a cause for anxiety.
On the whole we must conclude that the present routes are not very favour-
able to the infection of Asia, and it only remains to endeavour to foretell whether
future routes will be more so. On this subject Major Tucker has suggested
that ships from the Panama Canal may follow a more southerly route, calling
perhaps at Christmas Island instead of at Honolulu, and being, during the whole
voyage, in a climate favourable to mosquito life. Such a change of route would
not, however, be adopted unless it materially shortened the journey, which is
fortunately not the case. In this connexion it is important that ships crossing
the Pacific usually follow as far as possible what are called the Great Circle
sailing tracks, which are considerably shorter than the Mercatorial tracks,
and that those tracks often take them into latitudes much higher than those of
the ports between which the ships sail. For example, the shortest route between
Panama (latitude 9° N.) and Manila (latitude 14° N.) is a great circle track which
passes for about 1,000 miles of the journey above the 4Oth degree of north
latitude; and the shortest between Panama and Hong Kong (latitude 22° N.)
is a track which passes for about 800 miles of the journey above the 44th degree
of north latitude and afterwards touches Yokohama. The distance between
Panana and Hong Kong by that route is about 330 miles shorter than by a route
which on a chart on the Mercator Projection would be represented by a straight
line between the two ports. It will be apparent, therefore, that ships from the
Panama Canal will, in all probability, go northward to Yokohama not only on
account of the trade with Japan but because such a course is the shortest that
can be followed ; and it will be clear "that the fact that vessels follow those tracks
materially lessens the risk of the conveyance ot mosquitoes across the Pacific.
The climate of the present route from Salina Cruz to Tahiti for New Zealand
and Australia is, of course, favourable to mosquitoes, but I have. already men-
tioned the possibility that the canal may attract the traffic which now departs to
those countries from that port ; if this happens it will, in my view, lessen the
present risk that the ships may start their voyage with infected mosquitoes
on board.