=67. Placing the instrument.= The psychrograph should be located in a place where the circulation of the air is typical of the station observed. A satisfactory shelter will screen the instrument from sun and rain, and at the same time permit the air to pass freely through the perforations of the metal case. The form shown in figure 8 meets both of these conditions. A desirable modification is effected by fastening a strip about the cover of such depth as to prevent the sun’s rays from striking the case except when the sun is near the horizon. A cross block is fastened on the post of the shelter after being exactly leveled. The psychrograph rests upon this block, which is three feet above the ground in order to avoid the influence of radiation. The instrument is held in position by slipping the eye over a small-headed nail driven obliquely. It does not hang from the latter, but must rest firmly upon the cross block. The post is set to a depth that prevents oscillation in the wind, which is liable to obscure the record. In shallow mountain soils stability is attained by fastening a broad board at the base of the post before setting it. When two or more psychrographs are established in different habitats, great pains are taken to set them up in exactly the same way. The shelters are alike, the height above the soil the same, and the instruments all face the south. =68. Regulating and operating the instrument.= When two or more psychrographs are to be used in series, they must be compared with each other in the same spot for several days until they run exactly together with respect to per cent of humidity and to time. During this comparison they are checked by the psychrometer and so regulated that they register the proper humidity. When a single instrument is used alone as the basis to which simple readings may be referred, all regulating may well be done after the instrument is in position. This is a simple process; it is accomplished by obtaining the relative humidity beneath the shelter and at the proper height by a psychrometer. The pen hand is then moved to the proper line on the disk by means of the screws at its base. These are reached by removing the lettered glass face. The thumbscrew on the side opposite the direction in which the pen is to move is released, and the opposite screw simultaneously tightened, until the pen remains upon the proper line. Experience has proved that the record sheet should be correctly labeled and dated before being placed on the disk. In the press of field duties, records labeled after removal are liable to be confused. It is likewise a great saving of time to write the date of the month in the margin of each segment. Care is taken to place the sheet on the disk in the same position each time; this can easily be done by seeing that the sharp point on the disk penetrates the same spot on the paper. A single drop of ink in the pen will usually give the most satisfactory line.