mixture of crude and refined oils, which spreads better, or a mixture of oil and larvicide, are most generally used for the latter purpose. It is difficult, however, to get a good film unless all vegetation is removed from the water proposed to be covered, and even then wind may destroy the film. One gallon of crude petroleum per 1,000 square yards of breeding-surface is a rough allowance for each application, but local experiments are necessary for a more exact estimate. Thinner oil will cover a larger surface, but it evaporates more quickly, and the film is not so stable. Oil may be sprayed either by means of an ordinary rose-headed water-can or by a special spraying-apparatus.

A larvicide is more certain in its action than oil, but its use is limited by reason of possible harmful effect on man and beast or crops, although larvicides are available which are said to be harmless. The economical use of a larvicide requires intelligence, as a rough calculation is necessary to determine the quantity required for each collection of water to be treated.

Their

Oil and larvicides require to be applied to breeding-places about once a week; oil can with advantage be applied more often. use needs extraordinary care in order to be at all effective.

When adopting measures directed against larvæ the area to be treated should be cleared of all undergrowth and grass should be cut short, in order to facilitate evaporation and completely disclose all breeding-places.

REDUCTION OF ANOPHELINE MOSQUITOES BY ELIMINATION OF

BREEDING-PLACES.

It has already been pointed out that Anopheline mosquitoes are largely swamp or pool breeders, and for this reason, therefore, thorough drainage of a locality will get rid of Anopheline breedingplaces, and hence reduce Anophelines, and consequently malaria. The influence of drainage and improvement in cultivation on malaria was noted long before the part taken by Anopheline mosquitoes was known. This method of dealing with the problem presents at first sight a very satisfying solution, but on closer examination it is found to possess limitations.

All malaria-carrying Anophelines are not alike in their habits; some do not move far from their breeding-place, while others are able to travel considerable distances, up to a mile or more; some require particular conditions and are delicate, others are more hardy and are found under many conditions. The migratory habit of mosquitoes does not seem to have been noticed until recent years, and it would appear to be an occasional and not a regular habit,

probably peculiar to certain species and localities. As it is only practicable to drain a limited area to protect a particular population, it may happen that in a given case such a limited area will be insufficient for the purpose. On the other hand, the effect of a migratory swarm of mosquitoes in a drained area is not known, and it would seem probable that the swarm would migrate again or die out for lack of suitable conditions for life.

Thorough drainage in places having a large rainfall is by no means simple, although the difficulties have been overcome in the Federated Malay States, as will be described at some length later. Drainage as an anti-malarial measure is impossible in rice-fields, which must be covered with water for considerable periods.

Reliable statistics on the question of the distance needed between breeding-places and a population in order that the latter may be adequately protected are not numerous. On the flat lands of the Malay States it has been found that if breeding-places are

mile from habitations, very little malaria occurs among the inhabitants. This distance has been adopted as a desirable standard in connection with hill-land estates, but reliable definite figures will not be available for a year or two. The same distance has apparently been adopted in the Panama Canal zone for permanent towns, with satisfactory results, although here migratory swarms of mosquitoes have been noticed occasionally.

It is necessary to consider what is required of drainage as an anti-malarial measure. Water is a necessity for the propagation of mosquitoes, although all mosquitoes will not develop in any water, neither do they breed in water moving with a fair velocity, nor in water of considerable depth clear of vegetation. Clearly, therefore, it is essential that all land in a drained area shall normally be perfectly dry at the surface, and be rapidly freed from water even after heavy falls of rain; that drainage-water shall flow with a fair velocity in channels, free from obstruction, and that all bodies of still water shall be of some depth and entirely free from vegetation.

These requirements can be met on flat land, both in temperate and in tropical climates, and coincide with the usual requirements of agriculture; but the thorough drainage of hill-land in countries having a large rainfall presents many novel problems requiring much thought to attain satisfactory solutions, and demands great care in execution, particularly in the tropics, owing to the difficulties added by the habits of native populations. The methods adopted for work on hill-land under the Malaria Advisory Board in the Federated Malay States will therefore be described.

Ravines in the Malay States are invariably swampy along their bottoms and can only be laid dry by a continuous drain running

completely round the hill-foot, cut on a line at a little above the wetsurface level. This applies to even narrow ravines in which the wet surface is only a few feet wide. In wide ravines, or where the centre is much below the level of the hill-foot drains, a further drain or drains may be necessary between them. The foregoing rule is general for ravines with earth bottoms; where the bottom is of rock or has special geological features it must be dealt with accordingly.

It is of little use to provide adequate drainage if storms can wreck the system, and for this reason open earth-drains are of no use in localities subject to heavy rainfall. It has been found that the only way to construct permanent hill-foot drains is to use agricultural drain-pipes as subsoil-drains, and to provide outlet-drains of open masonry or subsoil-pipes to carry off their discharge. Only the ordinary flow is provided for in designing such drains, the surface being prepared to allow of flood-water during rains running off direct.

A general rule with regard to the capacity of subsoil pipe drains cannot be laid down, but if the pipes are proportioned to run not more than one-quarter full (one-fifth for main drains over 6 inches in diameter) during ordinary dry weather (at least 2 or 3 days after moderate rainfall), they will be sufficiently large to do their work during floods. It is not possible to calculate either the discharge during floods at a depth below the surface or the capacity of subsoil-drains under flood conditions. Observations have been made of drains discharging during floods, after water had ceased to flow off the surface, five times their capacity as given by the ordinary formula, neglecting the effect of the surcharge due to water saturating the soil above them.

The use of pipe drains less than 4 inches in internal diameter even for hill-foot drains is not advisable, as it is found that the smaller pipes are liable to choke. Three-inch drains are occasionally used where there is no danger of this.

The pipes are laid at a depth of 3 to 4 feet (or deeper on steep slopes) on a carefully graded trench-bottom, and covered with palmleaves, or long grass or leaves should palm-leaves not be available, and then filled over with earth. Clay should not be used. In clay soils the top few inches of soil is usually light enough for filling. Should the ground be very soft or fluid when draining, a layer of a few inches of broken stone or gravel is laid around pipes. The agricultural pipes are laid in the usual manner, close together, but unjointed. Special junction-pipes are used at connections.

Pipe drains have been laid at the heads of ravines on slopes of 1 in 3, and have satisfactorily withstood falls of as much as 6 inches of rain in as many hours.

All trees should be removed from the vicinity of pipe drains, as

and Figs. 2 cross sections of pipe drains.

Figs. 1 show the drain lines and cross section of a typical ravine;

long grass should not be allowed to grow up for the same reason. their roots may grow into and choke the pipes. Undergrowth and

Drains in long ravines must usually be cut from the lower end upwards, partly in order to get water away to allow of ease in working, and also because it has been found that after clearing, should drainage be commenced from the upper end, mosquito breeding-places, of a kind particularly suited to the worst malariacarriers in the Malay States, have been created at the lower end, with the consequence that an immediate temporary increase in the malaria of the locality has followed, instead of an immediate improvement. When the work is begun from the lower end the minor drains are completed as the work progresses, leaving only one open drain--the main channel-to be put in by work from the upper end. Thus, as the swamp is cleared it is at once dried, which is a great advantage in every way. Open masonry channels