HUMIDITY 43 



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 in- 

 fluence 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 lirmly 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 psy- 

 chrographs 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 re- 

 spect 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 instnmient 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 psychrojneter. 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 re- 

 moving 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. A thin line is read most accurately. If the pen 

 point is too fine, however, the ink does not flow readily, and the point 

 should be slightly blunted by means of a file. More often the line is too 

 broad and the pen must be carefully pointed. Occasionally the pen does not 

 touch the sheet, and it becomes necessary to bend the hand slightly. This 

 is a frequent difficulty if the records are folded or wrinkled, and conse- 

 quently the sheets should always be kept flat. 



