coughing the material is carried about ten feet. They demonstrated that masks of medium meshed gauze, two to ten layers thick, worn by the person coughing did not prevent the passage of infectious material into the air, but that a three-layer buttercloth mask was much more efficient. Haller and Colwell (44) used three distinct sets of experiments—one with the mask over the mouth of the patient, one with the mask over the plate, and the third with masks over both—and concluded that a five-layer mask made up of 24 × 20 mesh protected the plate in the second series of experiments. They suggested marking one side of the mask, so that it would always be worn with the same side out. Leete (45), in England, by a similar series of experiments concluded that a dry mask of six to eight layers of butter muslin worn by a contact would protect him against droplet-carried infections. Dannenberg (46) suggested making the gauze mask over a copper screen wire frame to give it shape and keep it away from the mouth, thus keeping it relatively dry. All observers agree that masks while dry are more efficient than they are after they have become moist. The efficiency of the mask has also been widely discussed from the clinical standpoint. Mink (47) in discussing their use at the Great Lakes Training Station said that he had no objection to the mask as it is “intended to be worn,” but that as it “was worn” by the medical corps men at the station 8 per cent. of those who used the mask developed influenza, as compared to 7.75 per cent. of those who did not; 30 per cent. of the dental officers at the station developed the disease in spite of the fact that they were all accustomed to wear masks during their work. In discussing the mask Vaughan (48) said: “With reference to the mask, I am strongly of the opinion that we have overestimated its value. * * * When I went to Camp Devens they were not using the mask. I called the doctors together and told them its use was not compulsory, but I said: ‘Every doctor who took care of cases of pneumonic plague and did not wear a mask died from it, and every man who cared for pneumonic plague cases and didn’t wear a mask did contract it.’” They were then allowed to choose for themselves. It has been pointed out that the epidemic dropped off at once in San Francisco with the universal compulsory use of the mask on the street, but it is also said that the epidemic in Los Angeles, which ran a course parallel to that in San Francisco and in which masks were only indiscriminately used, began to drop off simultaneously. While it is difficult to get at the facts, it seems that, provided epidemic influenza is carried through the air or by means of droplets, the universal use of masks should decrease the number of exposures. The claim has been made that masks merely tend to prolong the epidemic, and that susceptible persons develop the disease after the epidemic proper has passed. If the mask will protect the susceptible