20 



THE GARDENER'S ASSISTANT. 



and F, springs. Supposing the lowest portion 

 of the stratum c, or that between E and F, to 

 be 4 feet lower than the highest part of the 

 curve on each side of it, then when the whole 

 is completely saturated, the upward pressure 

 between E and F will be equal to about 250 lbs. 

 on the square foot. 



Some rocky strata, impervious to water, are 

 interrupted by fissures, as represented at ab 

 (fig. 807), which are called faults by miners. 



Fig. 807. 



Some of these fissures are occupied by sub- 

 stances which obstruct the passage of water, 

 others with those of a porous nature. Let A 

 represent a sandy stratum, B a clay one, and 

 C and D porous strata charged with water; on 

 reaching the fault at B, the water will collect, 

 and rising upwards so as to form a spring at 

 the surface, it will render the soil between B 

 and A too wet. The water, however, on reach- 

 ing the sandy fault at A, will pass down through 

 it to the porous strata, which, before the dis- 

 ruptions at A and B, had been continuations of 

 the strata C, D 



It will be observed that the strata are some- 

 times as irregular as the surface, but this is 

 not generally the case. On the contrary, they 

 mostly form inclined planes, so that when we 

 once get trace of a watery stratum, and can 

 ascertain the ratio of its slope, its depth from 

 the surface at any point so far as it extends, 

 and the place where it crops out to the surface, 

 can be pretty well determined. 



We must endeavour, in the first place, to 

 find the direction of the greatest slope or in- 

 clination of the stratum. This 



may be done by digging down * ; . ; ; v 



to it in three places, as at A, \ ""**•»./''/ I 



B, D (fig. 808), their position ' - -' /" |e 



being such as to form three ./ \ / I 



angles of a square. Then find /' / \ ; 

 the relative levels of watery j- 

 strata at these three points. 

 If D be the lowest, and A and B 

 be on the same level, the slope of the stratum is 

 direct from A B to c D. If B is the highest, and 

 A and D equally lower, the greatest slope will be 

 from B to c, crossing the diagonal at right angles 

 in 0. If B is the highest, A lower, and D the 



r 



Fig. 808. 



lowest, then the line of greatest slope will be 

 through some point between o and D. To find 

 this point, divide the line A D in proportion to 

 the slopes of A B and B D. Let that of the 

 former be 8 inches, and that of the latter 

 20 inches. Suppose the line AD is 226 feet 

 2 inches in length, and divided in the propor- 

 tion of 8 to 20, it will give 64 feet 7 inches 

 as the less, and 161 feet 7 inches as the greater 

 portion. It is evident that the greatest fall 

 will be nearer D than A; therefore the greater 

 proportion set off from A to D, or the less from 

 D to A, will mark the point through which the 

 line of greatest slope, B F, intersects the line 

 A D. Or, as the difference of level between 

 B and D is to the distance between these 

 points, so is the difference of level between 

 A and B to the distance from B towards D, 

 where the stratum will be at the same level 

 as at A. Stretch a line from A to the point 

 thus ascertained between B and D, and a line 

 across it anywhere at right angles will indicate 

 the direction of the greatest inclination of the 

 stratum. 



Suppose that from B to A there is a fall of 

 8 inches, and from B to D 20 inches, and that 

 the side of the square from B to D is 160 feet; 

 then as 20 inches : 8 inches : : 160 feet : 64 feet. 

 At the distance, then, of 64 feet, from B to E, 

 the stratum will have declined 8 inches, or as 

 much as from B to A; consequently A and E 

 will be on exactly the same level, and a line 

 drawn between them will be at right angles to 

 the line of greatest declivity. Accordingly, a 

 line B F, stretched from B and across A E at 

 right angles, will indicate the greatest inclina- 

 tion along this line. 



The depth of the strata below B should be 

 ascertained at say 50, 100, 200 feet or yards 

 apart, noting the difference between each and 

 that at B, in order to know the ratio of its 

 slope per 100 feet or yards. If it slope, for 

 instance, 6 inches in 100 feet, it is easy to find 

 the amount per 1000 or any other number, so 

 that at any point the depth below the starting- 

 point can be calculated. 



To ascertain the depth of the stratum at 

 say 800 feet from B: — Find the depth of the 

 stratum at that distance, which, presuming that 

 it slopes 6 inches in 100 feet, would be 4 feet; 

 also, how much the surface at that distance is 

 higher or lower than the surface at B; and let 

 it be supposed that at the latter it is 3 feet 

 above the stratum. If at the distance of 800 

 feet the surface is found to be 4 feet lower 

 than at B, then it will have sloped just as much 



