SOILS. CROPS AND FERTILIZERS. 



37 



Determining the Fall Along a Ditch. 



When a man wishes to determine 

 the fall along a proposed ditch he 

 sets up stakes 100 feet apart from the 

 outlet to the source, numbering them 

 0, 100, 200, 300, etc. He is then ready 

 to begin taking levels. He takes the 

 home-made level and places it be- 

 tween stakes and 100, as shown in 

 Fig. 2, sinking the upright firmly in- 

 to the ground as nearly perpendicular 

 as possible about half way between 

 the stakes and in line with them. If 

 it is windy, special care should be 

 taken to set the upright deep and 

 firm, as otherwise it will tremble too 

 much. He next places the level on 

 the cross-ipiece and makes it horizon- 

 tal by tilting and then using the 

 thumbscrews. Two men are required 

 to do the "levelling," A to sight and 

 B to hold the staff (or measuring 

 pole), and place a target (pencil or 

 something similar) across the staff 

 where directed. Fig. 3 shows them 

 at work. The staff is first stood on 

 the ground at stake and A sights 

 backward along the top of the level 

 and directs B to raise or lower the 

 target until it is in line with the 

 level, and when correct B makes a 

 note of the number of feet and inches 

 the target is from the ground. "When 

 this is done B moves forward to stake 

 100 and stands the staff on the 

 ground there and A, without moving 

 the level, turns around and sights 

 forward to the staff, directing B as 

 before. When the target is just level 

 with the instrument B again notes 

 the reading. 



In Fig. 2 the back reading was 4 

 feet 10 inches and the foresight 4 

 feet 1 inch. In both cases the target 

 was level with the instrument, con- 

 sequently the difference in reading 

 mu&t be due to the rise in the ground, 

 and, therefore, the amount of rise 

 must be 9 inches. The height of the 

 instrument is immaterial — the differ- 

 ence between the two readings wiH be 

 the same, whether the instrument is 

 on high or on low ground. When the 

 rise or fall from stake to stake 100 

 has been determined the level is next 

 placed about half way between stakes 

 100 and 200, and the rise or fall be- 

 tween them determined in the same 

 way. The level is next set between 

 stakes 200 and 300, and the same op- 

 eration repeated, and so on over the 

 whole course of the ditch. When this 



is completed, all the rises or falls, as 

 the case may be, are added together, 

 giving the total rise or fall. If there 

 are both rises and falls along the 

 same ditch, as frequently occurs 

 where a knoll or a hollow has to be 

 crossed, the difference between the 

 sum of the rises and the sum of the 

 falls will give the net rise or fall. 

 And when this is known, and also the 

 length of the drain, it is an easy 

 matter to find the rise or fall per rod 

 or per 100 feet. And this enables 

 one to decide whether he has fall 

 enough for underdrainage. The fall 

 in the ground surface, however, is not 

 always a test of whether a man can 

 underdrain, for he may put his drains 

 deeper at the outlet than at the 

 source and thus have more fall in the 

 ditch bottom than on the surface. 

 This we often find it necessary to do. 

 Ontario Agricultural College. 



Note that in six out of the eight 

 hundred feet sections there were 

 rise^, in the other two there were 

 falls. The six rises total 4 feet 11 

 inches, and the two falls total 11 

 inches, hence, on the whole there wa*? 

 a net rise of 4 feet from stake to 

 stake 800. 



The last column, "Elevation," needs 

 a word of explanation. In comparing 

 the altitude of different places, we 

 use the sea-level as a datum plane, 

 i.e., a given level of comparison. To- 

 ronto Bay is 250 feet, the Agricul- 

 tural College at Guelph 1,150 feet 

 ' "above the sea," from which we learn 

 by substraction that the College is 

 900 feet above the Bay. In a similar 

 wav we compare the elevations for 

 different points along a ditch, but in 

 surveying the latter we cannot use 

 the sea as a datum, for we do not 

 know how much stake is above the 

 sea, hence we must choose an arbi- 

 trary datum. 



In the example given we have 

 chosen it ten feet below the groimd 

 surface at stake 0. Then the eleva- 

 tion of the surface above this datum 

 plane at stake is 10 feet. Since 

 there is a rise of 9 inches to stake 

 100, its elevation will be 10 feet 9 

 inches, and so on with all other 

 stakes. To find the rise from any 

 one stake to any other, we have only 

 to subtract the elevations as given in 

 the last column. For instance, the 

 rise from stake to stake 800 is 14 

 feet, minus 10 feet, equalling 4 feet. 



