48 CLIMATIC CYCLES AND TREE-GROWTH 



When it became necessary to move the instrument to a locality 

 where a suitable window was not available, 10 electric lights in a row 

 were used, with a mirror behind and several thicknesses of ground 

 glass between the lights and the curve to spread the light evenly. 

 This is mounted on a table or stand, but it is planned to combine all 

 this equipment with an attachment which will permit the curve to 

 turn on its center through a horizontal angle, for by this means the 

 range of analysis can be greatly extended beyond the previous 32 

 years. This slanting of the curve can only be done when it is at 

 maximum distance from the lens, for the two ends would come in at 

 obviously different scales. To do this the whole illuminator will have 

 to turn on a central vertical axis. 



Track and carriage — The cyclograph track is 18 feet long (see 

 Plate 5, C), made of light beams well braced, carrying cross-pieces, 

 notched at each end to hold two lengths of i-inch round steel shafting 

 which serve as rails. The rails are 18 inches apart. The carriage has 

 two grooved wheels on one side to run on one rail and hold the align- 

 ment. On the other side is a single flat wheel.* The carriage holds a 

 vertical mirror 30 inches wide and 15 inches high, facing the illumin- 

 ator and the analyzing-box. Seen from the mirror, the former appears 

 slightly, but directly, above the latter. The carriage is moved by a cord 

 passing over a small wheel at the outer end and a drum with small spiral 

 groove about it at the observer's end. This drum has a handle within 

 reach of the observer as he sits at the side of the analyzing camera. 



Scale — The scale runs along the side of the track and the carriage 

 has a mirror and light so arranged that the observer may see the 

 lighted scale at any position of the carriage. A small telescope is 

 provided for reading the distant positions. The graduation is put on 

 from standardized curves, which are always kept on hand and measured 

 and tried from time to time. In dry climates all curves shrink per- 

 ceptibly and thus scales have to be watched. 



Range extension — The actual length of the track covers a range of 

 periods from 5 to 18 years. In order to increase this to 32 years, two 

 mirrors have been used, one fixed high above the track, throwing a 

 beam back toward the analyzing-box, and the other at the front of 

 the box in this beam, so placed that when it is raised in position it 

 catches the beam from the first extra mirror and sends it to the mirror 

 on the carriage, at the same time cutting off the direct light from the 

 curve to the carriage. This nearly doubles the maximum path of the 

 fight from the curve to the analyzing-box and increases the range of 

 periods tested from 18 years to over 32 years. 



Camera inclination — One bit of awkwardness remains in this 

 design, namely, the necessary change of slant of the camera-box when 



♦This same carriage was used on September 10, 1923, in photographing the total solar 

 eclipse from the University of Arizona station at Port Libertad, Sonora, Mexico, with a 40- 

 foot horizontal telescope. 



