Sentence:  In the Southern Ocean the winter is not so excessively cold, but the summer is far less hot, for the clouded sky seldom allows the sun to warm the ocean, itself a bad <WSD>absorbent</WSD> of heat: and hence the mean temperature of the year ... is low. 
Meaning of the word marked with <WSD> tags:  Anything which absorbs. (First attested in the early 18th century.)<ref name=SOED/>