Looks like it might be a natural spring that was "piped" so it flows into the basin so horses could drink and the outflow used to go into a small stream, but now goes into the city's sewers. Would the street happen to be Spring Street, or Road, or Alley??? Neat picture. A town where I worked had a central park across from the courthouse and a similar granite tank with a natural spring running into it. As many times as I've seen that structure, I know it has a date carved into the granite, but I can't remember it....