As indicated by the caption, this image shows the Triton fountain.
The caption accompanying this photograph reads as follows:
The Triton fountain, Stanford White's fabulous find for Harbor Hill, the focus of Guy Lowell's fashionable Italian garden, did not last very long beyond the architect's inglorious demise. Somewhere, there must be some photograph that shows it playing in this setting?
See Harbor Hill, Clarence Hungerford Mackay's Mansion.