152 MICHIGAN ACADEMY OF SCIENCE. 



In order to make the significance of contour lines more clearly under- 

 stood, a picture is here presented through the courtesy of the U. S. Geo- 

 logical Surve}', of a bit of coast, with a terraced valley between two hills 

 on the adjacent land; and beneath it is a contour map of the same 

 locality, on which the contour interval is 50 feet. 



If one imagines the land shown in the sketch, to be intersected by sev- 

 eral horizontal planes, fifty feet apart, the lines produced by the inter- 

 section of such planes with the surface of the land, would be contour 

 lines. These lines, if viewed in all parts from directly above, would show 

 the shape of the surface of the land. From the illustration it will be seen 

 that a contour map is a horizontal projection of contour lines; that is, 

 each contour line is dropped, as it were, to the plane in which the lowest 

 in the series is situated, which is usually the horizon of the ocean's sur- 

 face, and the figui'es placed upon it show the height of its previous 

 position. 



By comparing the above sketch with the map of the region it repre- 

 sents, it will be seen that where slopes are steep, the contour lines are 

 near together, and where the slopes are gentle the lines are wider apart ; 

 the vertical distance between any two adjacent contours being always 

 the same. In making surveys for contour maps, actual lines are not 

 marked on the land, but the topographer locates on his field sheet a num- 

 ber of points having the same elevation, and while occupying these sta- 

 tions, sketches the course of the lines connecting them. It is to the ac- 

 curacy of this freehand sketching controlled by a number of definitely 

 located points, that the expression and much of the value of the finished 

 map is due. 



It is by means of contour topographic maps of the nature just de- 

 scribed, usually drawn to a horizontal scale of about one mile to one 

 inch, or stsott f^f nature, but frequently of still larger scale, that most 

 enlightened nations, as previously stated, are striving to represent their 

 respective territories. No doubt the reader will enciuire: Why the wide- 

 spread demand for such maps among the people of the most progressive 

 countries? 



USES OF CONTOUR TOPOGRAPHIC MAPS. 



The advantages of contour topographic maps over the more familiar 

 diagrams of boundaries, roads, streams, etc., are due to the fact, as al- 

 ready stated, that they are representations of a given area in three in- 

 stead of two dimensions. From them vertical as well as horizontal dis- 

 tances can be measured. The value of the vertical element or the relief, 

 is very great, especially when combined as it is on all creditable contour 

 maps, with the results of accurate linear surveys. 



To attem])t to explain all Ihe aids to industrial, commercial and in- 

 tellectual development, furnished by contour topographic maps, would • 

 require more space than I have at command ; and besides, on account of 

 their faithfulness to nature, such maps create for themselves new and 

 previous to their appearance, unsuggested uses. In many branches of 

 nature study, they may be likened to the application of a previously un- 

 employed agency, such as the use of steam, electricity and compressed 

 air in the mechanical arts. In the hands of engineers, geologists, for- 

 esters, biologists, farmers, statesmen, and to teachers and students in 



