English: The illustration is taken from John Tyndall's introductory book on the physics of heat, Heat Considered as a Mode of Motion year 1869 edition. The apparatus on the right is a thermopile, which is a sensitive device for measuring temperature changes. In the upper part of the illustration, the box on the left is a locked container of compressed air that has been let stand for a while. When the valve on this box is opened, the compressed air rushes out of the box and moves towards the face of the thermopile. This causes the temperature at the face of the thermopile to decrease. In the lower half of the illustration, the object on the left is a bellows. The motion of the bellows takes in air, compresses it, and pushes the compressed air out through a nozzle towards the face of the thermopile. This causes the temperature at the face of the thermopile to increase. Discussed in the book on pages 42 to 47 and also page 28.