Place the feet upon the rests and press down each in turn in a forward direction. This movement rotates a toothed wheel clockwise. The teeth engage in the links of a chain and, by driving the chain forward, cause a second toothed wheel to the rear of the chain to rotate, also in a clockwise direction.

Draw the diagram that illustrates this description. Of what is it a description?

Useful as diagrams, charts and illustrations are, they are rarely a satisfactory substitute for words. It is difficult in a diagram, for example, to direct the reader's eye to the point at which you want him to start and to make him follow a series of instructions in the correct order. Words and illustrations should help each other to give the full meaning. What the diagram more clearly shows, the words need not.

There is the same difference between our choice of words for the description of a process and for an imaginative composition as there is between a diagrammatic drawing and an imaginative painting. What points of difference can you think of? This does not mean that you do not have to use your imagination in the writing of facts. You need to imagine yourself in the place of your reader (and you would do well to imagine also that your reader, while not an idiot, is not exactly quick-witted!). Then say to yourself: Have I started at the beginning? Have I gone point by point to the end? Have I rushed over any stage of the description? Above all, is it as short and concise as it can reasonably be? Have I chosen my words with accuracy and care?

A. Draw in diagrammatic form any piece of machinery you know well (a sewing machine or a racing car, for example), number the important parts in this way and name these parts in a key below or alongside your drawing. Choose three parts and give a clear account of the work they do.

B. Fifteen to twenty years ago there died a very amusing illustrator named Heath Robinson who specialised in drawing complicated and idiotic machinery made of bits of string and wire, springs and odd angles of metal, and so on. He was so famous that, when I was your age, any very home-made piece of machinery was described as a ‘Heath Robinson contraption’.