Quoting from an obituary of Mr. I.S. Cassin we know the following about him:
Isaac S. Cassin was educated at the famous Friends’school at Westown and under private tutors, and early manifested a talent for mechanics. He served an apprenticeship with Messrs. I. P. Morris & Co., of Philadelphia, and subsequently became, successively, engineer of the Spring Garden Water Works, of Philadelphia Gas Works, Chief Engineer of the Philadelphia Water Works, and chief engineer of the United States Mint in Philadlephia. Relinquishing for a time public office. Mr. Cassin reorganized the UNION HYDRAULIC WORKS, and was one of the most eminent water and gas engineers in the country, having built not less than fifty water works in prominent cities throughout the country.

As documented below fire hydrants have been found embossed with either I.S. CASSIN & CO. or J. THOMPSON & CO.; but none with UNION HYDRAULIC WORKS embossing.