Spirit level. This device is used to measure how level a surface is. It consists of a tube filled with a coloured fluid and a single air bubble. When the spirit level is placed on an even, flat surface, the air bubble remains between the two lines in the centre of the tube. However, when the surface is tilted, the air bubble will rise to the highest point in the tube, to the left if the tilt is to the right, and to the right if the tilt is to the left.