Section   C

    A buld-it-yourself solar still（蒸馏器） is one of the best ways toobtain drinking water in areas where the liquid is not readily available.Developed by two doctors in the U.S. Department of Agriculture, it’s anexcellent water collector. Unfortunately, you must carry the necessaryequipment with you, since it’s all but impossible to find natural substitutes.The only components required, though, are a 5’×5’ sheet ofclear or slightly milky plastic, six feet of plastic tube, and a container—perhaps just a drinking cup — to catch the water. These pieces can be foldedinto a neat little pack and fastened on your belt.

     To construct a working still, use a sharpstick or rock to dig a hole four feet across and three feet deep. Try to makethe hole in a damp area to increase the water catcher’s  productivity. Place your cup in the deepestpart of the hole. Then lay the tube in place so that one end rests all the wayin the cup and the rest of the line runs up — and out — the side of the hole.

      Next, cover the hole with the plastic sheet,securing the edges of the plastic with dirt and weighting the sheet’s center downwith a rock. The plastic should now form a cone(圆锥体) with 45-degree-angled sides. The low point of the sheet must becentered directly over, and no more than three inches above, the cup.

     The solar still works by creating agreenhouse under the plastic. Ground water evaporates (蒸发) and collects on the sheet untilsmall drops of water form, run down the material and fall off into the cup.When the container is full, you can suck the refreshment out through the tube,and won’t have to break down the still every time you need a drink.

What’s the  last step of constructing a working solar still