In one sentence, describe what the following article is about:

Hold the clinometer so the curved rim of the protractor is facing downward. Tilt the clinometer until you can look through the straw or paper tube and see the top of a tall object you want to measure, such as a building. You can use this method to measure the angle between you and the top of that object, or the object's height. Keep the clinometer steady in that position, until the dangling string becomes still. Calculate the angle between the midpoint of the protractor (90º), and the point where the string crosses the rim by subtracting one from the other. For example, if the string crosses the rim at 60º, the angle of elevation between you and the top of the object is 90–60=30º. If the string crosses the rim at 150º, the angle of elevation is 150-90=60º.  The angle of elevation will always be less than 90º since 90º is straight up in the sky. The answer will always be positive (greater than 0º). If you subtract the larger number from the smaller and get a negative number, just cross out the minus sign to get the right answer. For example, if you calculate that 60-90=-30º, the actual angle of elevation is +30º. The tangent of an angle is defined as the side of a right triangle opposite the angle, divided by the side adjacent to the angle. In this case, the triangle is formed by three points: you, the base of the object, and the top of the object. The "opposite" side from this angle is the height of the object, and the adjacent side is the distance between you and the object's base.  You can use a scientific or graphing calculator, an online tangent calculator, or a chart listing tangents for various angles. To calculate the tangent on a calculator, press TAN and enter the angle you found. If the answer is below 0 or above 1, set your calculator to degrees instead of radians and try again. If you want to find out how tall the object is, you'll need to know how far away you are from its base. Measure using a tape measure. If you don't have one, count the number of ordinary steps it takes to get to the object, then measure the length of one step once you find a ruler. The total distance is the length of one step multiplied by the number of steps you took.  Some protractors have rulers marked along the straight edge. Remember, the tangent of your angle is (object's height) / (distance between you and the object). Multiply the tangent by the distance you measured, and you'll be left with just the object's height!   For example, if the angle of elevation was 35º, and your distance from the object was 45 units, the object's height equals 45 x tangent(35º), or 31.5 units. Add your own height at eye level to your answer, since that's how far your clinometer was above the ground.

Summary:
Sight the top of a tall object through the straw. Measure the angle using the protractor. Calculate the tangent of this angle. Measure your distance from the object. Use your measurements to calculate the height of the object.