There is a very simple way of measuring the height of a water-tower，which we cannot measure by climbing．
Suppose，for example，that we wish to find out the height of the water-tower，AB，in a factory．We first of all go to where the water-tower is standing and measure a distance of，say，90 feet from it，in a straight line，marking the spot that is 90 feet from the water-tower．Then we take a stick，CD，and stand it in the ground at the spot we have just marked．
Let us suppose that the stick we are using is 4 feet in height．We now walk farther away from the water-tower in the same straight line as when we measured off the distance of 90 feet．We go from the water-tower until we come to a point E，where，with our head on the ground，we see the top of the stick and the top of the water-tower in the same straight line—that is，the top of the stick just covers the highest part of the water-tower．
We now have two triangles（三角形）and the proportion（比例）of the side CD to the side CE，in the smaller triangle，CDE，is exactly the same as the proportion of the side BA to the BE，in the larger triangle BAE．It is clear from this every schoolboy or schoolgirl can work out the height of the water-tower．
Suppose that the line CE is 5 feet．We know that the stick is 4 feet high and the length BE is 95 feet．From these measurements we have the simple proportion sum（算题）5 is to 4 as 95 is to AB．Now we can find out the height of the water-tower．
Trees and other high buildings can，of course，be easily measured by using the same way．
1．The height of the water-tower is __________．
A．76 feet B．95 feet C．90 feet D．180 feet
2．E is __________．
A．a point on the ground B．a straight line on the ground
C．the top of the stick D．the top of the water-tower
3．The triangle CDE must be __________．
A．below the triangle BAE B．in the triangle BAE
C．out of the triangle BAE D．over the triangle BAE
4．The proportion of the side CD to the side CE is __________．
A．bigger than the proportion of the side BA to the side BE
B．equal than the proportion of the side BA to the side BE
C．not as big as the proportion of side BA to the side BE
D．smaller than the proportion of the side BA to the side BE
5．From these measurements we have the simple proportion sum __________．
A．BA is to four as five is to ninety-five
B．BA is to ninety-five as four is to five
C．five is to ninety-five as BA is to four
D．four is to five as BA is to ninety
参考答案：1．A 2．A 3．B 4．B 5．B
