A clinometer is a device used in the past to measure the height of tall objects. It is based on the mathematical fact that a right angled isosceles triangle has two sides of equal length. Therefore if you stand at a 45 degree angle from a tall object the distance between you and the object is the same as the height of the object.
The girls fashioned their own clinometer using card, a straw, a length of string and a piece of blutac. They then used their clinometers to measure the heights of various objects in the school grounds.