The model in the courtyard

A scale model of a theatre was being taken from the workshop to a display in the library. The wooden base rolled freely on three wheels, but the fourth castor had locked at an angle and dragged across the ground. Outside the workshop, we had to control the model as it descended a steep ramp. The cobbles in the courtyard made the delicate sections tremble whenever the base jolted. A loose protective sheet caught the wind and pulled against our hands. When we finally reached the library, the entrance lobby was crowded with visitors leaving a lunchtime talk.

Describe the difficulties of moving the model to the library. You may support your points with brief quotations.