IRRIGATION. 169 



that lower le^. The difference in the level of the two legs will be accu* 

 rately indicated by tlie graduated scale on the cross bar. If, for instance, the 

 distance AD be one-third the length of one of the legs, (the angle formed by 

 the two legs being a right ani^le,) then a nioveuient of the plumb line half 

 an inch from the center, would indicate a diU'orence in the level of the two 

 points of the legs of about two inches. The following is a jjeneral rule fur 

 this purpose applicable to all cases with suflicient accuracy, where the ground 

 does vary greatly from a level. Multiply the distance AD by the number of 

 times the leg exceeds it in length, then, as fourteen is to twenty, sc is the 

 product to the dift'orence in the level of the two points of the legs. 



If the two points are therefore one rod apart, a cliannel may be expeditiously 

 and accurately laid out, so as to have a slope of two inches in a rod, by 

 placing the level in such a position that the plumb will fall half an inch Irom 

 the center if the distance AD is one-third of AB, or seven-tenths of an inch 

 irom the center, if AD is half of AB, according to the above rule. Success- 

 ive points are thus found by alternately bringing forward the legs of the in- 

 strument, each being two inches lower than the preceding. Small sticks are 

 driven into the ground at these points, and by them a uniformly descending 

 furrow is easily and accurately plowed for a distributing channel. If half 

 that slope only is needed, one-half the distance from the center of the gradu- 

 ated bar is to be taken. 



The preceding course of marking the channel is more particularly adapted to 

 uneven ground, where every rod of distance must be accurately known. But, on 

 large tracts of nearly level land, it Avill be easier to attach two sig/ils at the ends 

 of the cross bar, and take longer observations— a rod, marked at a hight equal to 

 the hight of these sights, being held at a distance by an assistant. 



Such a level may he made to close, like a pair of compasses, when not in ac- 

 tual use. If the points are accurately one rod apart, the operation of measuring 

 the distance is combined with that of leveling. 



Precautions Needed. — Irrigation, like every other farm operation of impor- 

 tance, needs to be conducted with care and skill. A want of judgment or proper 

 intelligence may in some cases lead to failure, or greatly lessen advantages. 



Practice has fully shown that too long a continued and heavy flooding of up 

 land plants is actually prejudicial to their growth. The plants should enjoy the 

 full benefits of both air and water. There is no better way of accomplishing this 

 object than to keep the water constantly passing over the surface in a tolerably 

 brisk current. It must not be so rapid as to wash away the soil, nor so slow as 

 to stagnate, or to drown the plants. It is only while water is in motion that 

 plants are enabled to draw from it to advantage, by successive fresh supplies, the 

 nutritive substances it contains in solution. A farmer accustomed to the appear- 

 ance of plants when in the most rapid and healthful state of vegetation, will de- 

 tect at a glance any injury which an overdose of water may occasion, when the 

 supply should be withdrawn. Excessive irrigation may also prove injurious 

 where it produces a water-soaked subsoil, the remedy for which should be drain- 

 ing. Indeed, so different in nature is a water-soaked and an irrigated soil, that, 

 while the former injures, the latter benefits; the former state resulting from a 

 want of draining, and the latter always proving most eminently beneficial on well 

 drained land, where stagnant water can never accumulate. 



Equal success has not always attended irrigation, in consequence of the differ- 

 ent quantities of enriching matter contained in different streams. The most val- 

 uable, usually, are those which have passed through a thickly populated country, 

 and have received and become saturated with different kinds of waste manures. 

 Hence the drainage of sewers, and the washings of roads, are particularly valua- 

 ble. Sometimes, however, there arc fertilizing substances derived from the soil 

 or the minerals it contains, which may not in the least disorder the water, and 

 yet be of very important benefit. These can only be known by their effects in 

 practice, or by analysis. But these considerations more particularly demand at- 

 tention in cool and moist climates, as in Britain, where the simple application of 

 water is unimportant. Hence, there autumn and winter irrigation is much prac- 

 ticed — the water being then charged with animal and vegetable matters which 

 have accumulated during the summer. In the United States, where a supply of 

 (361) ' n y 



