Harbour engineer. The son of an Edinburgh lawyer, Cay was educated at the University of Edinburgh where he won the highest mathematical prize, the Straiton Gold Medal, in 1856. He was a cousin of mathematician and physicist James Clerk Maxwell (1831 - 1879), and was best man at Clerk Maxwell's wedding.
Cay served his engineering apprenticeship between 1856 and 1858 in Belfast with James Thomson (1822-92), Lord Kelvin's brother. In 1861, he served as an assistant with B & E Blyth working on the Castle Douglas to Portpatrick Railway. Cay went on to specialise in the construction of harbours and, for many years, was the resident engineer at Aberdeen Harbour.
He was elected a Member of the Institution of Civil Engineers in 1872 and, ten years later, a Fellow of the Royal Society of Edinburgh.
