The sketch Bhows a Stephenson's thermometer screen, which consists of a box, either square or oblong, raised 4ft. from the ground. The box may have louvred sides, that is, the sides may be made in a similar way to wooden shutters for windows, thus allowing air to penetrate, but keeping out the direct rays of the sun. But it is preferable to have the louvred sides double, as illustrated in section by Fig. 2, and not single louvred. In strong winds, direct draught on the damp cotton surrounding the hygrometer wet bulb would produce undue evaporation, and give a lower temperature than would be given by the same thermometer when standing in still air of the same temperature. The double louvre minimises the risk of direct draught, and keeps the enclosed air as still as possible. The box is open below and has a wood partition through the middle upon which the thermometers may be fixed. The roof is sloped, and may be painted or covered with tarred felt. The size of the box is not important: but if it is made smaller than 3 ft. by 2 1/2 ft. by 2ft., it will be necessary to have a hinged door at each end through which to take the readings of the thermometers.