A geologist's slide kulb. 247 



between the values represented by the curves between which the point 

 lies, and its value may easily be estimated to within a degree. 



Another frequently recurring problem is one which, unfor- 

 tunately, is too often neglected by geological draughtsmen. In 

 drawing a geological section, the profile of the land surface is first 

 drawn from the data supplied by contour lines, determined heights, 

 &c. In a long section, if the same scale be used for horizontal and 

 vertical distances, the width of the section becomes inconveniently 

 small, and the surface irregularities become insignificant. It is, there- 

 fore, customary to exaggerate the vertical scale. For instance, we 

 often find a horizontal scale of 1 mile to the inch, and a vertical scale 

 of 1,000 ft. to the inch. In this case the vertical scale has been 

 exaggerated 5'28 times. This number 5'28 may for convenience be 

 called the factor of exaggeration. 



In order to determine the factor of exaggeration in any given 

 case, determine the number of feet (or yards) on vertical and hori- 

 zontal scales corresponding to 1 in. (or any other convenient unit) in 

 the diagram. Divide the number for the horizontal scale by that for 

 the vertical scale, and the result is the factor of exaggeration. This 

 is readily obtained by means of the logarithm scales of the rule. 



The result of exaggeration of vertical scale is to increase the 

 slopes of all surface features (hills and valleys). It should be obvious 

 to eveiyone that, in order to obtain a correct section, geological dips 

 should be increased proportionately. This is verj' frequently omitted, 

 with the result that sections are either "faked"* (an immoral pro- 

 ceeding on the part of a scientist), or else they convey an entirely 

 incorrect impression. 



Suppose in the right-angled triangle ABC the apparent dip of a 

 r\ geological structure in the direction of section 

 is equal to the angle ABC, and the factor of 

 exaggeration is, say, 3; it is obvious that to 

 obtain the angle which must be plotted in 

 order to correctly represent the structure on 

 our enlarged section we must produce AC to D, 

 making AD equal to three times AC. Join DB, 

 then the angle ABD is the angle required. 



If the apparent angle of dip be P, and the 

 angle to be plotted be Q, and the factor of 

 exaggeration be — 



tan.Q = n tan. P. 



This value can at once be obtained from the slide rule. 



Draw out the slide and reverse it end for end in the slide way. 

 This brings a logarithmic scale against the tangent scale. Place 1 on 

 the logarithm scale against the value of apparent dip on the 

 tangent scale, and read off on the latter the graduation against 



* Readers, please excuse the use of this slang term. 



