Probably he had been thinking of the whole of the simile in Cato, of which that is the concluding line; the sandy desart had struck him so strongly.

Thou hast frequented these sad desarts here; Thy time imployed in heedful minde I bear, The patient sufferance of thy former wrong, Thy poore estate and sharpe exile so long, The honourable port thou bor'st some time

The Guests are scattered thro' the land, For the Eye altering alters all; The Senses roll themselves in fear, And the flat Earth becomes a Ball; The Stars, Sun, Moon, all shrink away, A desart vast without a bound, And nothing left to eat or drink, And a dark desart all around.