An unusual simile for size, here referring to bubbles. Bubbles feature largely in Tarka the Otter, often blown by the otter himself as he swims beneath the water or as an expression of fear.
‘Near the bridge the bubbles rose as large as oak-apples.’
Source: Henry Williamson, Tarka the Otter: His joyful water-life and death in the two rivers, illus. C.F. Tunnicliffe (Harmondsworth: Puffin Books, 1976 (1927)), p. 18
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