The opinion seems to be: Cooper’s Hawk. I decided to delete a major portion of this previously published blog message. A lot of time and research went into preparation… but I don’t want to publish erroneous information. See my subsequent blog titled Cooper’s Hawk visitor.
Postscript, Tuesday, June 9, 2015, 6:30 AM: An answer arrived via email–from eBird–and I share it here.
**Lorraine – The bird in your photos looks like a Cooper’s Hawk. They are very localized breeding birds in your area and you may not run across them unless you are looking for them, or, as in this case, it visits your back yard. My guess is that either you had some water it was looking for or perhaps something that attracted a prey item (rodents, small birds, etc). Feel free to update your eBird submission as you see fit. Roadside Hawk is extremely rare in the US with only a handful of sightings. –Eric**
Lorraine comment: Yes, the fountain with fresh water was nearby. Yes, there are rodents and small birds in my yard.
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I’ve checked my several bird books–and the Internet–and haven’t identified my evening visitor. My “guess” is a hawk.
Three Quarters & Counting 
Sharpshink Hawk.
Why is this not a Cooper’s Hawk?
Your “mystery” bird is actually an adult female Cooper’s Hawk.
Cooper’s Hawk- note the size, eye color, bill size and shape, as well as what could be expected in the area.
It is an adult Cooper’s Hawk. You can tell it is a Cooper’s by its size, shape (an accipiter), and it’s dark cap.