For doing this ditching, each inspector had a gang of men under a competent foreman. In many instances, where the work was extensive and required unusual skill, such as tile-laying, the gangs especially experienced in this work were transferred from district to district. Often it is not possible to drain—for instance, a large swamp area, or where the locality is to be occupied so short a time that it is not economical to drain. In such an extensive engineering work as was being carried out on the Isthmus, the construction work was constantly interfering with the drainage and making pools and puddles which had to be looked after. In these instances, and many others of similar character, the use of kerosine oil was our only resource. The oil was distributed from a can on a man’s back, by a pump which he worked with his hand, forcing the oil through a nozzle to the place which he wished to affect. In general, we used crude oil of commerce, not because it was the best, but because it was so cheap that we could use it much more extensively than the rectified oil. It was so thick, however, that we had to thin it with various mixtures before we could spray it through the nozzle. This cost us a fraction over two cents a gallon, and we used about fifty thousand gallons a month over the hundred square miles of territory we were treating. It is not necessary to look after all collections of water in order that mosquito-breeding may be prevented. If the water is sufficiently deep and clear of grass, so that the fish can have free access, these fish will destroy all the larvæ, and mosquitoes will not develop. But all natural collections of water in a warm climate such as Panama have grass and algæ freely growing on its edges, and these obstructions protect mosquito larvæ, particularly the larvæ of the malarial mosquito, the anopheles. Gatun Lake, the large lake made by the damming of the Chagres River, gave us a great deal of trouble on this account during the last two years of construction. Not only would the anopheles breed freely around the edges of the lake, but wherever trees or vegetable matter were floating, there algæ grew freely and anopheles larvæ were found in abundance. We found that the anopheles larvæ on the Isthmus developed just about as freely in the mountains as they did in the lowlands. The clear mountain torrent seemed to be just as acceptable a home for the anopheles as the edge of the lowland swamp. At Panama, the temperature remains the same all the year round. The water of the mountain stream is sufficiently warm for his development, just as in the swamp. The stream, as it winds down the mountain, grows grass and algæ freely on both sides of its tortuous course, and here the larvæ of the anopheles find safe harbor. Into this grass and algæ oil will not spread. We had to find something that would dissolve in the water and poison the larvæ protected by the grass and algæ.