The prefix Fitz- is an old word for "son of", especially used for the illegitimate sons of royalty. Bear that in mind as we lay out this factoid.
We recently came across this exciting book, A Descriptive Catalogue of the manuscripts in the Fitzwilliam Museum, by Montague Rhodes James (yeah, that M. R. James). The Fitzwilliam Museum is the art and antiquities museum of the University of Cambridge and was founded in 1816 with the bequest of the library and art collection of the 7th Viscount FitzWilliam.
Ah! The name Fitzwilliam is in a Bellairs book, too, is it not? Not so much a person but a place name. It’s Fitzwilliam, New Hampshire, where Professor Childermass and Johnny Dixon discover the Childermass Clock and The Spell of the Sorcerer’s Skull begins. Incorporated in 1773 by Governor John Wentworth, the town was named for his cousin, William Fitzwilliam, 4th Earl Fitzwilliam.
Interestingly enough, the Viscounts FitzWilliams had no direct relationship with the Earls FitzWilliams.
You heard it here first. No need to throw a fit about it.