For instance, at one time we had railroad construction going on near Diablo Hill. This hill is surrounded by fresh-water swamps on three sides. We put a force of several hundred men to work on this. They were fairly comfortably housed in ordinary box cars, the doors and windows of which had been carefully screened with wire netting. These cars were located at the foot of Diablo Hill, on the edge of the swamp. No anti-malarial work having been done here up to this time, anopheles were very numerous. The swamp was so extensive that efficient anti-malarial work was not considered then practicable. Prophylactic quinin in five-grain doses was being taken by these men, but we knew from a recent experience in this same locality that infection was so severe that prophylactic quinin alone would not protect our men. We knew, also, that a certain number of mosquitoes would get by the screens every night. With a number of men living in a car and using one door, we knew that on the average that door would be open a large portion of the time. From inspections, we had found that there were always a considerable number of mosquitoes in the cars. Mr. Le Prince reasoned that while the mosquito-catcher could not by any possibility catch all the mosquitoes in every car every day, he would catch most of them. It was therefore extremely improbable that any individual mosquito would escape the mosquito-catcher for ten successive days. As it takes ten days from the time at which the female anopheles mosquito bites the man sick with malaria until she herself becomes able to transmit the disease, no mosquito would live long enough under these conditions to become disease-bearing. And in practice this proved to be the case. The mosquito-catcher went through the cars every day and caught all the mosquitoes he could find, and continued this day after day as long as the cars remained in this swamp. The force was kept here for several months without suffering appreciably from malaria. This was the more impressive from the fact that just before we had our negro employees in cars on the edge of the swamp, we had kept a considerable force of marines for several weeks camped in tents on top of Diablo Hill. These marines suffered very severely from malarial fever, very few of them escaping. The tents not being screened, we could not carry out the same method of catching infected mosquitoes which was so successful in the case of our negro laborers, a couple of hundred yards distant from this same camp. One of the medical officers reported to me one day that, under a mosquito-bar which he had kept over a sick man during the preceding night, he caught in the morning quite a large number of anopheles—about fifty, as I recall it. We used this method more and more during the later years of our work on the Isthmus.