1026 



THE IRRIGATION -AGE. 



THE PRIMER OF HYDRAULICS* 



By FREDERICK A. SMITH, C. E. 



4. Properties of Gases. 



a. General deductions. 



Gases are bodies in which the molecules possess no co- 

 hesion whatever, and tend therefore to separate from each 

 other. Thus it requires force to keep the particles of any 

 gas or vapor together, such as a closed vessel, or, as in the 

 case of the atmosphere, the force of gravity, which prevents 

 our air from leaving the earth. 



b. Density of Gases. 



All gases and vapors possess weight, but there is much 

 difference in the weight of different kinds of gases. The 

 lightest gas known is hydrogen, and as for fluids and 

 solids, water is taken as the unit density, so for gases 

 and vapors hydrogen forms the base and its density is called 

 1 ; then the densities of other gases are multiples ; for 

 instance, the density of oxygen is 16, of nitrogen 14, of car- 

 bon dioxide 23, etc. The density of air is 14.44, which means 

 that one cubic ft. of air weighs 14.44 times as heavy as 

 hydrogen. 



c. The Pressure of the Air on the Surface of the Earth. 

 The atmosphere surrounds the earth like a mighty 



ocean many miles in thickness, and the weight of this mass 

 of air presses on the surface of the earth. That this pres- 

 sure is everywhere was first proven by Torricelli in the fol- 

 lowing manner. He took a glass tube DC (see Fig. 70), 

 which was closed at D, filled it with quicksilver, closing the 

 end C with his thumb, and immersed the end C under the 

 surface of quicksilver contained in a vessel AB ; then with- 

 drawing his thumb the quicksilver in the tube dropped a dis- 

 tance until the distance to the surface AB of the quicksilver 

 amounted to 30 inches ; the fact that the quicksilver did rot 

 run out of the tube is accounted by the pressure of the air 

 on the surface AB. And as the 30 inches of mercury just 

 balance that pressure it shows that the pressure of the air 

 on the earth is equal to that of a quicksilver ocean 30 inches 

 thick, and as quicksilver is 13'/2 times heavier than water it 

 is seen that if instead of quicksilver water would be sub- 

 stituted in the tube DC, then the pressure of the air on the 

 surface AB would balance a column of water in CD equal 

 to iy/2 times 30 inches, or nearly 34 ft. This principle ac- 

 counts for the air pressure lifting water in pumps and 

 shows also the limits of such lift, the max- 

 imum of which at sea level would hardly 

 reach 30 ft. 



The air pressure diminishes rapidly in 

 higher elevations because the thickness of 

 the atmosphere decreases as well as the den- 

 sity, the density at sea level being ordina- 

 rily a maximum. On high mountains the 

 density of the air and the air pressure is 

 perceptibly less and pumps will therefore not 

 lift water as high as in lower altitudes. 



d. Compressibility of Gases. 



If a bent glass tube EC, as in Fig. 71, 

 with the short arm C closed, is filled up to 

 the line AB with quicksilver, then there is 

 a certain amount of air imprisoned in the 

 short arm of the tube. If now quicksilver is 

 added in E carefully, it rises in both arms, 



Fig. 70. 



and when it reaches a point D, 30 inches above C, it is found 

 that the original volume of the air in BC has been reduced ]/2, 

 and likewise the pressure on the air has been doubled, as in 

 addition to the original air pressure the weight of 30 inches of 

 quicksilver has been added; if more quicksilver is poured into 

 E until the top of it at is 60 inches above the top of the 

 quicksilver at G, then it will be found that the volume of the 

 air in G is l /z of the original, and the pressure on that 

 ;iir is three times the air pressure. This experiment explains 

 the important law of Boyle, which states that the volumes of 

 gases are inversely proportional to the pressures ; thus a 



cubic ft. of air under a pressure of 10 atmospheres (150 

 Ibs. per square inch) will occupy only' the space of 1/10 

 of a cubic ft. Insersely, when the pressure 

 is relieved the gas expands without limit. 



e. Buoyancy. 



Bodies immersed in gases lose as much 

 weight as the volume of the gas weighs 

 which they displace. In this regard gases 

 are similar to fluids, and the principle is 

 vital in the navigation of the air. Balloons 

 tilled with a gas lighter than air will rise in 

 the air when the weight of the displaced air 

 is greater than the combined weight of 

 balloon and gas with which it is filled. As 

 hydrogen is the lightest substance, it forms, 

 the most desirable gas for filling balloons 

 as it has a greater buoyancy than any other 

 gas. 



/. Evaporation in Air. 



When the surface of air is in direct con- ~- 

 tact with fluids evaporation takes place, i. e., 

 portions of the fluid assume the gaseous state 

 and this continues until either all the liquid 

 is evaporated or the air has become saturated. Thus the air 

 can only hold a certain amount of the vapor of any particular 

 fluid ; this capacity is, however, increased by an increase in 

 temperature. 



LIGHT IN RELATION TO TREE GROWTH. 



The Secretary of Agriculture has just issued Bulletin 

 12 of the Forest Service on "Light in Relation to Tree 

 Growth." The bulletin is designed to show the benefits 

 derived by the tree by light from all angles, particularly 

 that reflected on the roots. 



In introducing the subject the bulletin states: "Light 

 is indispensable for the life and growth of trees. In com- 

 mon with other green plants a tree, in order to live, must 

 produce organic substance for the building of new tissues. 

 Certain low forms of vegetable life, such as bacteria and 

 fungi, do not require light. They exist by absorbing or- 

 ganic substance from other living bodies; but the higher 

 forms of plants manufacture their own organic material 

 by extracting carbon from the air. The leaves, through 

 the agency of their chlorophyll, or green coloring matter, 

 absorb, from the air carbon dioxide, and give off a nearly 

 equal volume of oxygen. The carbon dioxide is then 

 broken up into its elements and converted into organic 

 substances which are used in building up new tissues. 



"Light also influences transpiration, and consequently 

 the metabolism of green plants. It influences largely 

 the structure, the form and the color of the leaf, and the 

 form of the stem and the crown of the tree. In the forest 

 '.t largely determines the height growth of trees, the rate 

 at which stands thin out with age, the progress of natural 

 pruning, the character of the living ground cover, the 

 vigor of young tree growth, the existence of several- 

 storied forest, and many other phenomena upon which the 

 management of forests depends. A thorough understand- 

 ing, therefore, of the effect of light upon the life of indi- 

 vidual trees, and especially on trees in the forest, and a 

 knowledge of the methods by which the extent of this 

 effect can be determined, are essential for successful cul- 

 tural operations in the forest." 



*Copyrighted by D. T. Anderson. 



AN EXTENSION OF TIME GRANTED. 



A considerable number of water users on the Mini- 

 doka irrigation project, Idaho, have found it impractica- 

 ble to make payments as required by their contracts, as 

 this has been the most difficult or critical season encoun- 

 tered on this project. The farms are still new and have 

 not been fully subdued and planted in crops. 



The Secretary of the Interior has therefore consid- 

 ered it wise to give a little further extension to delinquent 

 entrymen who have tried to make payment and has made 

 the proposition that if the directors of the Water Users' 

 Association will verify the fact that certain of the water 

 users have tried to comply in good faith with the require- 

 ments of the Reclamation Act, and that they will pay all 

 charges for building, operation and maintenance, which 

 should have been paid on or before April 27, the secretary 

 will defer cancellation until Dec. 31, 1911. 



This requires the Water Users' Association to assume 



