THE IRKIGATION AGE. 



THE PRIMER OF HYDRAULICS* 



By FREDERICK A. SMITH, C. E. 



B 



d. Capillary Attraction. 



This means if a tube is placed into a fluid which "wets" 

 said tube, there is an attraction between the walls of the 

 tube and the fluid and the latter rises in the former as 

 indicated in Fig. 67. Let AB be a glass 

 tube immersed in a tumbler containing 

 water ; then the water rises in AB above 

 the level CD in the tumbler ; this rise 

 is proportional to the diameter of the 

 tube and is. greater the smaller the di- 

 ameter is. 



This phenomenon of capillary at- 

 traction is very important in nature. The 

 growing of plants and trees is accom- 

 Fig. 67. plished by the fibers and cells drawing 



their nourishment from the soil and sending it up to the 

 various portions of the plant by virtue of capillary attrac- 

 tion. 



Also the fact that a wick draws oil from a retainer up 

 to the flame of the light is due to this fact, the wick acting 

 as a number of capillary tubes. The evaporation of surface 

 moisture in soils is caused by capillary attraction, as the 

 pores in the soil act as capillary tubes and carry the mois- 

 ture to the surface, when the dry air evaporates the water 

 and the' pores in the soil draw up further water. This 

 effect is destroyed by breaking up the surface of the soil 

 (mulching), which destroys the tubular texture of the soil 

 and stops rapid evaporation. This principle is made use 

 of in arid countries and is the principal factor in dry agri- 

 culture. 



c. Capillary Repulsion. 



When a tube is immersed in a fluid 

 which does not "wet" the tube, a re- 

 pulsion is taken place as indicated in 

 Fig. 68. For instance, let EF be a glass 

 tube immersed in quicksilver, then the 

 height of the quicksilver in the tube will 

 be lower than in the larger vessel, and 

 the fluid is curving away from the walls 

 of the tube and the vessel. The depres- 

 sion depends on the size of the tube and 

 is greater the smaller the diameter of EF. 



Fig. 68. 



/. Buoyancy. 



If a block of wood is placed into a vessel containing 

 water it is observed that the block sinks down a certain 

 depth and then remains suspended, which phenomenon is 

 ordinarily termed swimming or floating. If instead of a 

 block of wood a block of stone is used the stone will sink 

 to the bottom of the vessel; when raising such stone under 

 water it is found, however, that it seems much lighter, 

 and in the act of lifting it from the water it appears to 

 get heavier as it emerges from the fluid. Upon careful 

 investigation it is found that every solid substance immersed 

 in water loses as much weight as the water weighs which 

 is displaced. This principle is called buoyancy, and applies 

 to all fluids. If the weight of the displaced fluid is greater 

 than the weight of the submerged body, as in the case 

 of the block of wood, then body rises to a point so that 

 the weight of the body equals the weight of the displaced 

 fluid, and the body is then said to be floating. If the weight 

 of the displaced fluid, however, is less than the weight 

 of the immersed body, then the body will sink to the 

 bottom of the vessel although its weight has been diminished 

 equal to the weight of the displaced fluid. 



The principle involved is illustrat- 

 ed in Fig. 69, in which the vessel 

 contains water to the line BC ; if a 

 block DEFG is immersed, then ac- 

 cording to the law of pressures in 

 fluids the pressure on top of the face 

 DE is equal to the weight of the 

 water column DEHI; the pressure on 

 the bottom face is equal to the weight 



B 



of the water column GFHI, hence the upward pressure on 

 the face GF is greater than the downward pressure on 

 DE by the difference GFHI DEHI DEGF, which is 

 the weight of the displaced water. The side pressures P 

 and Q are equal and opposite so they balance each other. 

 The upward pressure, then, will, if it is greater than the 

 weight of the block DEFG, lift the block until the upper 

 part of it rises just high enough above the surface of the 

 water so that the displaced water weighs exactly as much 

 as the block; then the upward pressure equals the weight 

 of the block and the downward pressure of the weight 

 of the block equals the upward pressure of the water which 

 produces equilibrium and the body floats. 



If the weight of the block DEFG is, however, greater 

 than the displaced water the force of gravity overcomes 

 the upward pressure of the water and the body sinks to 

 the bottom although with its weight diminished to the amount 

 of the weight of the displaced water. 



If instead of water other fluids are considered, the only 

 difference there is is due to the density of the fluid, i. e., 

 the relative weight of the fluid compared with water. Thus 

 if instead of water quicksilver is used, which is 13J4 times 

 as heavy as water, the buoyancy effect will be much greater, 

 and a solid block of steel, which quickly falls to the bottom 

 of a vessel filled with water, will float on quicksilver ; on 

 the other hand a block of wood floating on water will sink 

 to the bottom of a vessel containing oil or alcohol. 



g. Specific Gravity. 



Different substances have different weights per cubic inch 

 or cubic foot. A cubic ft. of water weighs 62.4245 Ibs. 

 at its greatest density of 39.1 F. ; from this temperature 

 it expands as it gets colder or warmer ; at freezing point 

 :i2 F. 1 cubic ft. of water weighs 62.42 Ibs., and at the 

 boiling point, 212 F., it weighs 59.76 Ibs; at 60 F. it 

 weighs 62.37 Ibs., which is a good average for perfectly 

 pure water. The water we have to deal with, however, is 

 seldom chemically pure, but contains other substances in 

 solution which tend to make it heavier, and for this reason 

 02.4 Ibs. is much nearer the average weight of a cubic ft. 

 of water. 



By specific gravity of any substance is meant a figure 

 which indicates the ratio of the weight of a unit volume of 

 that substance compared to the same unit volume of 

 pure water. Thus the specific gravity of quicksilver is 13.5 : 

 this means that one cubic inch of quicksilver weighs 13.5 

 times as much as a cubic inch of pure water. 



To find the specific gravity of any substance, first weigh 

 the substance in air; let its weight in air equal W ' ; next 

 weigh the substance in water and let the observed weight 

 equal iv ', let S equal the specific gravity sought, then : 



W 



Fig. 69. 



Copyright by D. H. Anderson. 



W w 



To illustrate : It is required to find the specific gravity 

 of a piece of ore; it is first weighed in air, and assume 

 its weight is 12 ounces; next the same piece of ore is 

 weighed under water, and we will assume its weight then 

 is 10 ounces ; then according to above formula find S by 

 substituting the observed values : 

 12 12 



S = --- . = = 6 



12 10 3 



which gives 6 as the specific gravity of the substance in 

 question. 



This principle is very important and one easily applied. 

 A table of specific gravities of the more important substances 

 appears elsewhere in this book. 



h. Compressibility of Water. 



The common impression is that water is not compressi- 

 ble; it is, however, slightly so, the coefficient being .000003 

 per Ib. pressure per square inch, so that it would take a 

 pressure of a million pounds per square inch to reduce its 

 volume 3/10. So for all practical purposes it may be said 

 water is incompressible. 



How little this compressibility of water does amount to 

 may be inferred from this fact, that even at a depth of a 

 mile in the ocean, where the pressure is something enormous, 

 5,280 X 625 = 333,000 Ibs. per square foot, or nearly 2,292 

 Ibs. per square inch, yet the weight of a cubic foot of 

 water is only half a pound more than it is on the surface. 



Hence it is perfectly permissible to ig/iore the com- 

 pressibility of water. . 



