power-driven  pump  with  suitable  hose  attach- ments supplies continuous water pressure during drilling. One type of rig uses a block and tackle or  a  tripod  for  controlling  the  tools  and  casing. Larger rigs have a mast and hoisting block and use  engine  power  for  handling  casing,  drive weight,  and  pump.  Water  is  led  into  the  well through  a  small  diameter  pipe  and  forced downward  through  the  drill  bit  against  the bottom  of  the  hole.  The  stream  loosens  the material,  the  finer  portion  of  which  is  carried upward  and  out  of  the  hole  by  the  ascending water. During the drilling, the jet or drill is turned slowly to ensure a straight hole. Casing is sunk as  fast  as  drilling  proceeds.  In  softer  materials, by using a paddy or expansion drill, a hole may be  made  somewhat  larger  than  the  casing.  The casing   then   may   be   lowered   a   considerable distance  by  its  own  weight.  Ordinarily  a  drive weight is needed to force it down. As a rule, one size of casing is used for the entire depth of the well.  It  is  difficult  to  drive  a  single  string  of casing beyond 500 to 600 feet by this method. If a well is sunk much deeper, an additional string of  smaller  size  must  be  used.  In  fine-textured material the hole often may be jetted to the full depth  and  the  casing  inserted  afterward.  The  wall of the hole becomes puddled by the muddy water so  it  will  stand  alone. A driven  well   is   constructed   by   driving   a pointed screen, or drive point, and attached pipe directly  into  a  water-bearing  formation.  The finished  well  consists  of  a  series  of  lengths  of  pipe fitted at the upper end with a pump and the lower end with a sand screen through which the water is  admitted.  The  drive  point  consists  of  a  per- forated pipe with a mild steel point at its lower end  to  break  through  pebbles  or  thin  layers  of hard material. As the drive point is driven down, succeeding sections are screwed into place. These sections continue until the water-bearing forma- tion is reached. The pump then is attached, and after the well has been developed, it is ready for use. Drive point wells usually range in diameter from  1  1/4  to  2  inches,  but  larger  sizes  up  to  4 inches also are made. The larger sizes, although of  greater  weight  and  more  difficult  to  drive,  have the advantage that deep-well pumps can be used when necessary. The smaller sizes, because of their lesser  weight  and  greater  portability,  are  valuable for  determining  the  depth  of  water-bearing  for- mations  and  for  test  yields  at  shallow  depths.  The depth  of  the  well  is  limited  by  the  formations encountered and by the type of pump available. For small wells, the ground water level must be within  25  feet  of  the  surface  because  suction pumps generally are used. If small self-priming centrifugal pumps are used, the lift must be less than 25 feet. If 2-inch or larger pipes are used, it is possible to lift water from a greater depth by installing  a  cylinder-type  pump  near  the  water level. The  following  conditions  are  necessary  for successful driven wells. The formation into which the point is being driven must not be too hard and compact. The distance to ground water must not exceed the lift of the pumps available. The water- bearing  formations  must  have  moderately  high permeability to provide adequate yields in small- diameter  wells.  The  wells  must  be  developed properly  to  obtain  sufficient  water. Chief  disadvantages  against  general  use  of driven  wells  are  as  follows.  Construction  is laborious and slow when tightly compacted soils are  encountered.  Driving  is  destructive  to  well equipment;  points  frequently  are  stripped  of mesh;  pipe  is  bent  and  broken.  Couplings  fre- quently  are  belled  by  the  force  of  the  hammer blows.  Belled  joints  always  leak  air  and  either render  the  well  useless  or  seriously  impair  the  yield of water. Yields are small from any one well point. As  many  as  five  points  connected  in  series  may be required to operate a power pump to capacity. Successful   construction   of   driven   wells depends  upon  close  observation  and  correct interpretation of events (occurring while driving) by  the  well  driver.  Accurate  interpretation  of  such details as the penetration made with each blow, the drop and rebound of the monkey, the sound of  the  blow,  and  the  resistance  of  the  pipe  to  rota- tion enables the experienced well driver to deter- mine  the  character  of  the  materials  being penetrated.  An  approximation  of  the  geological section of the well can be obtained by recording these observations. Study of the logs for successive wells,  coupled  with  a  study  of  the  results  obtained from  each  well,  assists  in  developing  trained well drivers with each successive well. Although a well site may have been properly selected, the strata correctly interpreted, and the presence  of  water  accurately  judged,  wells  may fail to yield water merely because they have not been  pumped  to  clear  the  fine  sediment  from around  the  screen.  When  the  presence  of  water is suspected, a simple test is to pour water into the well. If the screen is in dry sand, the water sinks  downward  and  seeps  into  the  formation,  but if  the  screen  is  in  saturated  sand,  the  level of the added water remains nearly stationary or quickly sinks to a static level. Also the quantity 9-12