The HYDRA FLOAT Automatic Stage System of the early 1970's was a hydraulic fly system manufactured by HYDRA FLOAT Stage Systems, Inc. Installed at Broadway's new Uris (now Gershwin) Theatre, the system was completely removed soon after the opening attraction. The word was that the linesets were prone to drift, that is, they could not maintain trim height. The attraction was Via Galactica, which opened in November, 1972 and played one week.
Here is the HYDRA FLOAT brochure from the Sweet's catalog of 1972.
To see a video about the 1983 Gagon LaForest's (GALA) Vertilift hydraulic fly system, click here.