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 the 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 .or of an inch, the time of each 
exposure is 72 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 selagraph. 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 1 cm. wide and 59 cm. long. The opening in the box: 
