4733 HYDROGRAPHIC MANUAL PaGE 408 



cide because the paths of descent and ascent will be at different places, and the temperatures at equal 

 depths may differ. The Bourdon type of bathythermograph may be provided with a Nansen bottle 

 clamp, which contains a mechanism that can be tripped by a messenger to raise the stylus from the 

 glass slide when the instrument reaches its lowest point of descent. This eliminates any temperature 

 trace being made during ascent. 



» 

 4733. Projector and Slides 



The glass slides are very small, 1 by 123^2 inches, and the temperature trace must be enlarged at 

 least four times for satisfactory use (see C in fig. 88) . 



A special projector is used to throw an enlarged image of the trace on a ground-glass screen where 

 it may be traced with pencil or pen on a translucent calibration graph. The slide holder in the projector 

 is constructed to hold the glass slide in the same relative position it is held in the bathythermograph 

 and no reference marks are needed on the slide. The special projector is expensive and is not essential, 

 for the shdes may be projected in any 35 mm commercial enlarging camera with a good lens, obtainable 

 from any camera supply store. When the latter is used the slides have to be projected on a calibration 

 graph, where the temperature trace is adjusted to thermometer temperatures plotted thereon as refer- 

 ence points. These thermometer temperatures are taken by reversing thermometer, simultaneously 

 with the bathythermograph measurement. 



A calibration graph must be constructed for each bathythermograph from a complete series of 

 simultaneous observations with the instrument and a reversing thermometer. The depths are plotted 

 as ordiuates and the temperatures as abscissas. The graph consists of a system of vertical parallel 

 lines crossed by a system of curved lines with a radius equal to the length of the stylus arm connected 

 to the temperature element (in the Bourdon type) , or the length of the bimetallic reed (in the bimetal- 

 lic-reed type), multiplied by the number of times the trace is to be enlarged. An arbitrary spacing 

 between the curves is chosen such that the graph will cover the maximum depth range of the instru- 

 ment. The spacing of the vertical lines is determined by projecting on the graph the various slides 

 for which the ranges of temperature have been accurately obtained from simultaneous observations. 

 The distances on the graph for various temperature differences resulting from the comparisons are 

 then averaged and the correct spacing determined for degree intervals. The abscissas of the graph 

 are made for the range of temperature of the instrument. A suitable size calibration graph may be 

 made on 8- by 10}^-inch paper to provide for enlarging the trace eight times when projected. After 

 the calibration graph has been constructed by the field party it may be sent to the Washington Office 

 for reproduction (see 482) . 



After calibration, a simultaneous thermometer temperature at the surface and a depth measure- 

 ment of the lowest part of the temperature trace suflBce theoretically to correlate the temperature trace 

 to the calibration graph. Practically, additional simultaneous thermometer temperatures are needed 

 near the midpoint and lowest part of the trace. 



A box containing 100 glass slides is furnished with each batthythermograph. The slides are made 

 of noncorrosive glass, smoked on one side by a special process. After a slide has been used and the 

 temperature trace has been permanently recorded on the calibration graph, the smoked coating may be 

 removed and renewed for use a second time. The sUde should be thoroughly cleaned in a bath of 

 carbon tetrachloride, or a similar liquid, to dissolve the thin film of oil which is the base to which the 

 smoke adheres while it is submerged in water. After the slide has been cleaned it should be wiped dry 

 with a clean cloth. Rendered skunk oil is the base for the smoked coating and should be applied with 

 the finger in a very thin uniform coat on one side of the slide. Skunk oil is not readily obtainable at all 

 places but it may generally be purchased at old established pharmaceutical companies in large cities. 

 When it is not available a thin coating of vaseline or petroleum jelly may be substituted. The slides 

 are smoked best over a coal gas flame to obtain a light coating of carbon particles, but this type of 

 flame is not usually available on board ship. The best substitute is cigarette lighter fluid in an alcohol 

 wick lamp. The glass slide is held over the flame, oil-coated side down, and the oil baked until its 

 glossiness has disappeared and the entire surface of the slide has a dull appearance. A piece of wire 

 may be bent into a shape to hold the glass slides over the flame. They should not be held over it too 

 long because the glass may crack if it is overheated. 



