It is of considerable importance to use a screen the mesh of which is sufficiently fine. Where mosquitoes exist, the screen should be of such fineness that at least sixteen, or better eighteen meshes be in each inch of the gauze. Where it is absolutely certain that mosquitoes are not to be feared, the spaces may be somewhat larger--but always of such size as will prevent the entrance of the smallest fly. _Air-space Required._--It is of much importance from a hygienic standpoint that the rooms of dwellings should be sufficiently large. The height should never be less than eight feet, and the living-room should be made as large as circumstances will permit. Bed-chambers should contain at least 1,000 cubic feet of air space for each adult, with somewhat less for children, though it should never be forgotten that the more the better; this means that each person should have the equivalent of a room which is at least 10 x 12 x 9 feet. _Heating._--Americans are extravagant in the matter of heating to a degree that astonishes the average foreigner, and it is by no means sure that we do not go to unhygienic extremes in this direction. It is not, perhaps, true that the excessive heat itself could be considered as especially hurtful, but it is too often the case that the conditions required to secure the degree of heat preferred by us are incompatible with proper ventilation, and hence are to be condemned. It is generally considered that the temperature of living-rooms should be somewhere about 70 deg.F.; for many persons this is lower than would be entirely comfortable, and as a consequence our houses in the winter are frequently kept nearer 80 deg.F. than the figure just given. The reader should be urged to see to it that, at whatever temperature his habitation is kept, a sufficient amount of ventilation be secured. There are many different methods of heating, the most satisfactory of which are by means of hot water or steam; a modified form of the latter is the so-called vapor method, which in recent years has proven extremely satisfactory. Hot air, supplied by a furnace is also extensively used, and for the reason that by this method fresh air from the outside is constantly brought into the house, it is theoretically to be commended; practically, however, a considerable difficulty is experienced in securing an equable distribution of this heat throughout the various parts of the house, and as a consequence it has not achieved the popularity that it would otherwise have done.