South of 3° N., there are extremely important rivers, the first, in order, being the Asahan, which drains the lake of Toba, and is navigable by steamers for a portion of its length, next the Panei (Bila and Barumun), with a very wide mouth, then the Rokan, over 120 m. long, and flowing into Malacca straits through a wide, muddy estuary, is navigable for 7o m. inland; the Kampar, a very long river, navigable for ocean steamers for 18 miles, the Indragiri, which is navigable for ocean steamers as far as Kulachenako, and for river steamers as far as Cheranti. The Jambi, which springs from the peak of Indrapura, is the most beautiful and largest of all the rivers of Sumatra, having a maxi mum navigability of just under 500 m.; the Musi is the only one which bears comparison with the Jambi. Ocean-going ves sels can reach Palembang, 54 m. from its mouth, steam launches and motor boats navigate the river for 200 m., and small boats for 33o miles. Below Palembang the Musi splits up into a number of channels which spread out amidst a vast unhealthy swamp covering an area of some 4,600 m. These subsidiary channels slowly deposit their suspended quota of mud among the man groves, thus gradually creating new land, and slowly the whole coast-line is advancing sea-wards and tending to fill up the Malacca straits and join Sumatra to the Malay peninsula. But this ten dency may be counteracted by a sinking of the land.
On the coast, there are small semi-circular bays, but few large indentations, the principal being Koninginne bay, which, in Emmahaven, gives a good harbour, and Tapanuli bay (Sibolga).
North of Tapanuli and south of Indrapura none of the indenta tions provides immunity from the south-west monsoon. The north coast has precipitous cliffs crowned with dense vegetation in some parts ; in others sandy beaches, and well cultivated plains, whilst the bay of Oleh Leh gives shelter for shipping. The southern coast consists, mainly, of two large indentations, Lampong bay and Semangko bay, the eastern shore of the former being mountainous and little indented, whilst the western is much cut up; the centre of the bay has a good harbour (Teluk Betong). Semangko has a mountainous coast-line, with little indentation. The whole of the east coast is formed of morasses and sandbanks, which break up into innumerable islands, large and small, particularly about the central portion, and the coast line is deeply indented in many places and very irregular. The indentations are not permanent, the coast is constantly advanc ing, and it is difficult to establish practicable ports or landing places. Belawan, in the north, is the best harbour, and other harbours, also on the northern part of this coast, are Sigli, Lho Seumawe, Langsa, and Tanjong Balei. Sumatra has a chain of islands off the west coast which rise from the edge of the sub marine platform and serve as a kind of outer barrier. They have an area of 5,769 sq.m., several are thickly populated and some are of considerable size, whilst many are of volcanic origin and most have rocky, reef-bound coasts, dangerous of approach.
The climate of Sumatra resembles that of Java, and is hot and extremely moist, but in most eastern coastal dis tricts and often for many miles inland the heat is tempered by cooling land and sea breezes, the wind generally being north east by day and south-west by night all the year round. The wind system of north Sumatra differs from that of the greater part of the Dutch East Indies, the north-east monsoon blowing from December to March, and the south-west monsoon from May to October. During the south-west monsoon the strong squalls from the north-west known as "Sumatras," accompanied by thunder, lightning, and rain, blow in the Malacca strait, and they have been known to occur during the north-east monsoon. On the west coast the influence of the north-east and south-west monsoons is felt as far south as 4° N. ; between 4° N., and 2° N. there is a region of calms and light variable winds, which have a westerly tendency from March to November, when the north west monsoon is blowing along the remainder of the western coast, i.e., from 2° N. to 5° 57' S., and an easterly tendency from May to September, the period of the south-east monsoon on the western coast. Southern Sumatra has the highest temperatures, and the mean annual temperature, for the lowlands, is about 80°. At Toba, in the Battak country (3,772 ft.), this is 69.6°,
and higher altitudes show a corresponding reduction. On the whole, temperature in Sumatra is slightly higher than in Java. March, April and May are the hottest months; January and February the coolest, but the difference does not exceed 2°. The different wind distribution causes a variation of the seasons. Thus in north Sumatra October is the wettest month and Febru ary and March are the driest, whilst elsewhere the wettest months are December, January, and February, and from July to Septem ber is the dry period. Accordingly the rainfall varies consider ably. The annual average rainfall for north Sumatra is 95.71, for east Sumatra 106.27, and for west Sumatra 122.32 inches. The west monsoon gives the heavier rainfall and the fall is accentuated in west Sumatra by the high mountains.
Notwithstanding the proximity of the island to Java the fauna of Sumatra shows a greater resemblance to that of Borneo than to that of Java, this being particularly noticeable as regards the fauna of the east coast of Sumatra, whilst that of the west coast and the islands adjacent is more allied to the Javan fauna. The orang-utan, common to Sumatra (in the north east), and Borneo, is unknown in Java ; the siamang (Hylobates syndactylus) is found in Sumatra only; there are ape species common to Borneo and Sumatra, also the elephant, tapir and Malayan bear, all of which are unknown in Java, though, on the other hand, Sumatra shares the tiger with Java, whilst it is un known in Borneo. The one-horned rhinoceros of Java is unknown in Sumatra, which shares a two-horned variety with Borneo; the wild ox, or banteng (Bos sundaicus), known in Java, seems to be lacking in Sumatra, which has, however, a species of antelope, (kambing-utan), found only in the loneliest districts of the moun tains. The Sumatran fauna also includes the apes Hylobates agilis, Semnopithecus melalophus, Cercocebus cynomolgus and Macacus nemestrinus, fox-nosed monkeys (Tarsius); the slow loris (Nycticebus) ; tupai (Tupaja) ; and the flying maki, or lemur (Galeopithecus volans) ; the flying fox, the kerbau, or buffalo (Bos bubalus), the Malayan deer (Cervus muntjac); wild dog; wild pig; Sumatran hare (Lepus netscheri) ; and civet cat. Species of birds exist in Sumatra which are unknown in Java—the great Argus pheasant, the fire-backed and ocellated pheasants, crested partridge, small Malacca parrot (Psitticus incertus), great helmeted hornbill (Buceroturus galeatus), pheasant ground cuckoo (Carpococcyx radiatus), rose-crested bee-eater (Nyctiornis amicta), and the great and green-crested gapers. Other Sumatran birds include the woodpecker, pigeon, and trogon. Insects, which also show a marked difference from those of Java, comprise the beautiful Papilio memnon butterfly, the Papilio coon, and other species of this section, the Kallima paralekta (which attaches itself to and is able to imitate the leaf of a certain plant so closely as to secure itself from the attacks of enemies), flocks of Cyrestes, Melantis suradeva, a new species of Amnosia, Eurhinia Alva, one of the prettiest species of Ecophoridae, two specimens of the curious genus Homopsyche, and the moth Pentacitritus transversa. Lizards, large and small, frogs, tortoises and turtles abound, crocodiles frequent the mouths of rivers and are also found up-stream, there are many varieties of poisonous snakes, the rocks around the coast yield many kinds of shell-fish, and the sea, ponds, and rivers numerous sorts of edible fishes. The striking difference in species between the fauna of Sumatra and Java led A. R. Wallace to assert that the first severance of Sumatra from Java is very ancient ; that since the epoch of the first (volcanic) disturbance several dis tinct elevations and depressions may have taken place, and the islands may have been more than once joined with each other or with the mainland (of Asia), and again separated, and that successive waves of immigration may thus have modified their animal productions and led to those anomalies in distribution which are so difficult to account for by any single operation of elevation or submergence.