thwaite. the rivers had been deepened by the scour; in one case (the River Mr. HebblePavuna) in 3 months the channel was deepened 1 metre, and the surrounding lands were completely drained. In the low swamps between the rivers, canals 7, 12, and 15 metres in width, and of depths ranging from 2 to 4 metres, had been made. These canals were made with dredgers cutting their own flotation, the process of making them being the following. The forest was first cleared to a width of 50 to 76 metres, the trees being cut down to within 40 to 50 centimetres of the ground. As much of the timber as was necessary for fuel for the dredgers was cut into firewood, stacked, and left along the route; the rest of the timber and the brushwood were then burned. A centre width of 2 metres more than the finished width of the canal was then cleared of roots, the large roots having been to some extent, though not by any means always, removed by blasting with cheddite. Dikes of fascines and brushwood were now made on each side of the route, 5 metres clear of the finished width of the canal. The dikes were in general made 1 metre wide by 1 metre to 1 metre high. Stakes 3 to 4 inches in diameter were driven with a wood mallet, 80 centimetres apart longitudinally, in two rows; the intervening space was filled with fascines and brushwood, and the rows of stakes were tied together with steel wire (No. 11 gauge), laced across the top of the brushwood to prevent spreading. These dikes were constructed cheaply, and were sufficiently strong; they only fell when the ground was very soft and the banks of the canal fell in, and with them, of course, the dikes. The work was then ready for the dredging. The dredgers for the 12- and 15-metre canals were ordinary ladder dredgers with the ladder projecting well forward through the bow of the vessel. The stern of the dredger was held in a fixed position by wire ropes led off clear from the winches, and fastened to trees outside the clearing. The bow of the vessel was similarly secured, but as the work proceeded the bow winches were operated and the dredger swung from side to side, thus cutting the flotation of the dredger to the full width of the canal. A rope was led out from the bow, made fast about 300 metres ahead of the work, and kept taut on the winch, the dredger thus advancing automatically as the dredging proceeded. The spoil was tipped from the buckets into semicircular steel shoots, into which water was pumped continuously to assist the flow, the spoil running down the nearly horizontal shoots on to the ground behind the dikes. The dikes were tight enough to retain the spoil, though the water drained through, and though they were by no means permanent, before they were destroyed by the weather the spoil had become con[THE INST. C.E. VOL. CO.] E Mr. Hebble-solidated and would stand alone. In the narrow canals (7 and thwaite. 10 metres wide) a special dredger was used. In this the whole of Mr. Hodge. the ladder, buckets, top work, and the shoots were on a rotating platform, the ladder projecting beyond the bow, as with the other dredgers. The hull of the dredger, with a beam only 1 metre less than the finished width of the canal, remained in a fixed position, being held by spuds depressed into the bottom of the canal. The ladder rotated through a small angle about a vertical axis, the buckets thus moving through a horizontal arc, and dredging the canal to the desired width. This machine was ingenious, but it was not so practical a tool as the other. The engines, pumps, and winches were all on the hull, and the connections with the rotating platform were necessarily complicated. With the fixed ladder dredgers the progress had been 10 to 20 metres forward per day; the progress with the rotating machine was much less. On one part of the Canal da Santa Cruz the ground was high and the dredger could only work at high water, as at low water the shoots came in contact with the high banks. This limited the available workinghours, and little progress could be made. A dam with a sluice was therefore made behind the dredger; at high water this sluice was closed, the water was retained in the canal at the level of high water, and the dredger was enabled to work through the whole of the day. The work was commenced in May, 1911, and was still proceeding. It was being done by a firm of Continental contractors, who obtained the work in public tender, and was directed by a Commission of Brazilian engineers and paid for by the Brazilian Government. Where the work was completed malaria had almost entirely disappeared. During the progress of the work on the Canal da Santa Cruz there was so much fever among the workmen that it was decided to remove the camp from the work to the town of Merity, where the work had been completed, the men to be taken out to the work each day on a ballast-train. When, however, the clearing 75 metres wide and 6 kilometres long had been made, and before any draining had been begun, the conditions were so much improved, simply by the admission of sun and air, that it was possible to re-establish the camp on the site of the work, with practically no malaria among the men. Mr. L. P. HODGE remarked that ever since the original idea that malaria was due to bad air was dispelled by the investigations of the late Dr. Beauperthuy, Major Sir Ronald Ross, Sir Rubert Boyce and others, measures for destroying mosquitoes and draining their breeding-grounds had received considerable attention in British Guiana. The Author dealt more particularly with the treatment of hilly country intersected with ravines and possess- Mr. Hodge. ing good natural drainage. The conditions in inhabited British Guiana were entirely dissimilar. The coast lands formed an alluvial flat, mostly below sea-level, which necessitated the sea being kept out by continuous dams through which intertidal drainage was afforded by sluices. The soil was mostly impermeable clay, and owing to the generally uniform level, the filling of depressions presented considerable difficulty, which had usually to be surmounted either by enlarging existing drainage-trenches or digging additional ones. So great was the dearth of suitable filling that house-refuse, largely vegetable, had had to be utilized in many instances, each layer being sprayed with an insecticide if it could not be covered the same day with a top dressing of earth. The main trenches, being fairly deep and having more or less flow when the sluices were open, were fairly free from larvæ, particularly when they were kept weeded and contained the larva-eating small fish known locally as the "koker belly." A good deal of bushing had been done since the late Sir Rubert Boyce's last visit, in 1909, mostly with more zeal than discretion, it being often overlooked that bushes and shrubs were with certain exceptions not breeding-places, but only shelters for mosquitoes. The indiscriminate removal of large trees was much to be deplored. Apart from their grateful shade, their extensive evaporation materially assisted drainage. The roots also, as they grew, lifted the soil above them, and the root-area of each tree was a natural drainage system, the elevation of the top soil being generally clearly noticeable immediately around the trunk, thus lessening the rain-sodden area in which Anophelines would breed. He would go farther and advocate the planting of suitable trees in marshy land to effect drainage. The trees selected should be free from low branches and not liable to be overgrown by epiphytes, especially water-holding ones. The coco-palm for utility and the flamboyant for beauty were suitable trees to plant, the elevation of the ground around their trunks being generally well defined, while they afforded plenty of ventilation and were noticeably free from creepers. The destruction of mosquitoes was provided for by law in British Guiana (Ordinance No. 19 of 1910). The liability to keep water-storage receptacles mosquito-proof was imposed on the owner, while that of keeping premises free from temporary breeding-places, such as broken bottles, old tins, and shells, fell on the occupier. Proof of the finding of mosquito-larvæ was sufficient evidence of an offence, and the maximum penalty was $24 (£5). British Guiana depended upon roof rainfall for drinking-water, the usual storage Mr. Hodge. receptacles being wooden vats of 1,000 to 5,000 gallons capacity. Before legislation practically made effective screening compulsory, vat-covers were generally of boarding supported by bearers resting on the staves. The mosquito-proofing of a vat thus covered was an expensive and troublesome operation, the wire gauze having to be carried all round it and over the ends of the projecting bearers. In order to reduce the cost, and at the same time render the screening more effective, he had designed the cover shown in Fig. 11, which was approved by Sir Rubert Boyce and had since been generally adopted in the Colony. The cover, which was in two pieces, resting directly upon the staves, enabled any opening to be detected immediately, while the quantity of brass-wire web Staves, Fig. 11. AERATION MOLE Brasa per sq. in. G4greenheart heart Bas. G&T 2° Overlay, HALF HORIZONTAL SECTION 3 4. FEET was reduced to that required for aeration. A further improvement was the substitution of a removable strainer (Fig. 12), which also did duty as a manhole, thus obviating the removal of the cover for cleaning purposes, so that covers were now generally made in one piece. The pipes led over the strainer-box should have a free joint, so that they might be easily lifted or turned aside when required. The brass-wire web should have 20 × 20 = 400 meshes to the square inch. A larger mesh, 18 x 18 = 324 per square inch, was specified in a pamphlet issued by the Colonial Office in March, 1906, for the prevention of yellow fever, but was not found absolutely effective in British Guiana. The minimum strainer-area for the effective aeration of the rain-water in the tanks had not yet been determined by direct experiment, but that afforded by the strainer in Fig. 12, the clear dimensions of which would be 14 inches x 8 inches 112 square inches, had answered for the largest Government vats. The same type of cover had also been tried in iron, using 22 S.W.G. plain galvanized sheets and 1inch inch angle-bar stiffeners. Such covers were not much more expensive than wooden covers, and were considerably more durable. As to concrete gutters, the By-laws of the City of Georgetown provided that they should have an initial depth of not less than 6 inches, an internal width of not less than 9 inches at the top and 6 inches at the bottom, a minimum fall of 2 inches per 100 feet, and weep-holes not less than 1 square inch, at not less than 2 inches above the bottom, and at intervals of not less than Mr. Ilodge. 10 feet. This guttering was generally of 3-inch concrete laid continuously in situ, thus avoiding openings or settlements at joints. It was not found in Georgetown that the weep-holes, which materially assisted to drain the surface soil, harboured mosquitoes to an appreciable extent. This might be, however, simply because better breeding-places were available. The extensive increase of rice-cultivation in British Guiana had undoubtedly greatly increased Anophelines. On the other hand, considerable attention had been given to mosquito-prevention and to the use of quinine, the Government selling quinine at Post-Offices at cost price. The effect of the use of quinine, assisted by better drainage, in lessening the mortality from malarial fever might be gauged by the difference in the death-rate of the estates and the villages on the east bank of the Demerara River, given in the last Report of the Government Medical Officer of Health. The figures were (per 1,000): |