CLARK: HARVARD DEEP-SEA THERMOGRAPH. 521 



The light for the photographic work is produced by a small electric 

 lamp operated by dry cells. After traversing a peculiar optical sys- 

 tem necessitated by the general shape and compass of the apparatus, 

 it passes through the glass thermometer-stem in a parallel beam and 

 falls upon a film. The temperature is afterwards measured from the 

 height of the shadow whicli the mercury column casts on the film. 

 The length of exposure, and the intervals at which exposures are made, 

 are governed by a clockwork which causes the lamp to light at the 

 proper time, and afterwards moves the film along to prepare for the 

 next exposure. An intermittent record like this is preferable to a 

 continuous one, because it simplifies the time record on the film. 

 Before the apparatus is put overboard its clock should be compared 

 with the observer's watch. It will then be known at what times on 

 the watch the records are to be made, and at such times the operator 

 will note the depth as indicated by the amount of wire paid out. If 

 the temperature gradient is small, or if great accuracy is not required, 

 the thermograph may be lowered slowly and continuously. If the 

 greatest accuracy is desired the operator should lower it quickly after 

 each exposure and then allow it to remain at its new level until the 

 next record has been made. 



The photographic apparatus, lamp, batteries, films, thermometer, 

 and clockwork taken together constitute the works of the thermograph. 

 They are enclosed in a strong water-tight steel shell, called the flask. 

 The works, Plate 1, fig. 1, were made by C. L. Berger and Sons, Rox- 

 bury, Mass. The length without thermometer is 10 inches, and the 

 diameter 2-| inches. In fig. 2, Plate 1, the clockwork cover, front 

 casting, and thermometer have been removed. In all of the figures 

 a given part of the apparatus is always referred to by the same letter. 



Photographic Unit. 



The photographic unit, except for its front casting, is shown, fig. 4, 

 Plate 2. It is made up of four brass castings, called front, hack, top, 

 and bottom respectively, so fitted togetlier as to enclose a light-tight 

 cavity, and contains the lamp, batteries, optical system, and films. 

 The front casting is shown, fig. 5, Plate 2. Fig. 3, Plate 1 is a cross- 

 section of the photographic unit, the cross-hatched areas being the 

 sections of the front and back castings. The film is shown passing 

 between the spools a, a over the roller b, and the thermometer-tube 

 is shown at c. The optical system is contained in the front casting. 

 The battery-cells are contained iu the holes d, d. 



