Fig. 7.—Jetting Outfit.
U. S. Geological Survey, Water Supply Paper, No. 257
1. Simple Jetting Outfit. 2. Jetting Process. 3. Common Jetting Drill. 4a and 4b. Expansion Bit or Paddy. 5. Drive Shoe.
Methods of drilling in rock up to depths of 20 feet are described in Chapter XI under Rock Drilling. For deeper holes percussion, abrasive, or hydraulic methods as used for deep well drilling must be employed.
CHAPTER III
QUANTITY OF SEWAGE
17. Dry weather Flow.—Estimates of the quantity of sewage flow to be expected are ordinarily based on the population, the character of the district, the rate of water consumption, and the probable ground-water flow. Future conditions are estimated and provided for, as the sewers should have sufficient capacity to care for the sewage delivered to them during their period of usefulness.
18. Methods for Predicting Population.—Methods for the prediction of future population are given in the following paragraphs.
The method of graphical extension. This is the quickest and most simple of all. In this method a curve is plotted on rectangular coordinates to any convenient scale, with population as ordinates and years as abscissas. The curve is extended into the future by judgment of its general tendency. An example is given of the determination of the population of Urbana, Illinois, in 1950. Table 4 contains the population statistics which have been plotted on line A in Fig. 8 and extended to 1950. The probable population in 1950 is shown by this line to be about 21,000.
The method of geometrical progression. In this method the rate of increase during the past few years or decades is assumed to be constant and this rate is applied to the present population to forecast the population in the future. For example the rate of increase of population in Urbana for the past 7 decades has varied widely, but indications are that for the next few decades it will be about 20 per cent. Applying this rate from 1920 to 1950 the population in 1950 is shown to be about 17,800. It is evident that this method may lead to serious error as insufficient information is given in the table to make possible the selection of the proper rate of increase.
| TABLE 4 | ||||||||||
|---|---|---|---|---|---|---|---|---|---|---|
| Population Studies | ||||||||||
| Year | Urbana, Illinois | Population of | ||||||||
| Population | Absolute Increase for Each Decade | Per Cent Increase for Each Decade | Decatur | Danville | Champaign | Kankakee | Peoria | Bloomington | Ann, Arbor Michigan | |
| 1850 | 210 | 736 | 5,095 | 1,594 | ||||||
| 1860 | 2,038 | 1828 | 85.6 | 3,839 | 1,632 | 1,727 | 2,984 | 14,045 | 7,075 | 5,097 |
| 1870 | 2,277 | 239 | 10.5 | 7,161 | 4,751 | 4,625 | 5,189 | 22,849 | 14,590 | 7,368 |
| 1880 | 2,942 | 665 | 22.6 | 9,547 | 7,733 | 5,103 | 5,651 | 29,259 | 17,180 | 8,061 |
| 1890 | 3,511 | 569 | 16.2 | 16,841 | 11,491 | 5,839 | 9,025 | 41,024 | 20,484 | 9,431 |
| 1900 | 5,728 | 2217 | 38.7 | 20,754 | 16,354 | 9,098 | 13,595 | 56,100 | 23,286 | 14,509 |
| 1910 | 8,245 | 2517 | 30.5 | 31,140 | 27,871 | 12,421 | 13,986 | 66,950 | 25,786 | 14,817 |
| 1920 | 10,230 | 1985 | 19.4 | 43,818 | 33,750 | 15,873 | 16,721 | 76,121 | 28,638 | 19,516 |
