52 THE HABITAT 



a screw fixed to the lid of the outer cylinder, the drum holding the sensi- 

 tive paper is made to travel endwise down the outer tube, one-eighth of an 

 inch daily, so that a fresh portion of the sensitive surface is brought into 

 position to receive the record." The instrument is driven by an eight-day 

 clock placed in the base below the drum. The slit is covered by means of 

 a flattened funnel-shaped hood, and the photographic strip is protected 

 from rain by a perfectly transparent sheet of celluloid. The detailed structure 

 of the instrument is shown in figure 12. This instrument may be obtained 

 from Lander and Smith, Canterbury, England, for $35. 



In setting up the sunshine recorder, the axis should be placed in such a 

 position that the angle which it makes with the base is the same as the 

 altitude of the place where the observations are made. This is readily done 

 by loosening the bolts at either side. The drum' is removed, the celluloid 

 sheet unwound by means of the key which holds it in place, the sensitive 

 strip put in position, and the sheet again wound up. Strips of a special sensi- 

 tive paper upon which tlie hours are indicated are furnished by the makers 

 of the instrument, but it has been found preferable to use solio strips in 

 order to facilitate comparison with the standards. The drum is placed on 

 the axis, and is screwed up until it just escapes the collar at the top of the 

 spiral. The clock is wound and started, and the outer cylinder put on so 

 that the proper hour mark coincides with the index on the front of the base. 



As a sunshine recorder, the instrument gives a perfect record, in which 

 the varying intensities are readily recognizable. Since the cylinder moves 

 one-half inch in an hour, and the slit is .01 of an inch, the time of each 

 exposure is 'J2 seconds. This gives a very deep color on the solio paper, 

 which results in a serious error in making comparisons with the standard. 

 On account of the hood, diffuse light is not recorded when it is too weak 

 to cast a distinct shadow. It seems probable that this difficulty will be over- 

 come by the use of a flat disk containing the proper slit, and in this event 

 the instrument will become of especial value for measuring the diffuse light 

 of layered formations. The celluloid sheet constitutes a source of error in 

 sunlight on account of the reflection which it causes. This can be prevented 

 by using the instrument only on sunny days, when the protection of the 

 sheet can be dispensed with. 



81. The sciagraph. This instrument is at present under construction, 

 and can only be described in a general way. In principle it is a simple 

 photometer operating automatically. It consists of a light-tight box pref- 

 erably of metal, which contains an eight-day lever clock. Attached to 

 the arbor of the latter is a disk 7 inches in diameter bearing on its circum- 

 ference a solio strip i cm. wide and 59 cm. long. The opening in the box 



