William's second marriage on 14 Nov 1770 to Isabel Alexander (1735-1780,possible cousin of Wm), produced many children, one of which is (id 78211810): Jacob Alexander(1779-1851) (the grandson of this Jacob Alexander)

This memorial is for Williams 2nd wife - Isabel Alexander.

Her father in law is Jacob Alexander. This is what is written on his memorial:

Jacob Alexander(1709-1792 *His will was signed 19 Jul 1778 and proven on 14 Nov 1792) who was a Scottish Immigrant born in County Down, Ireland and immigrated to the United States to escape bitter British persecution for practicing their deeply felt Presbyterian principles.

This family settled in/early founders of Great Cove Ayr, Fulton County, Pennsylvania (Amazing historic stories/books, settlement, War, Indian Battles) & Early Alexander land patents.

This Jacob was the father of many, (uncertain of number) including the following:
dau Martha Alexander (b.abt 1737),
Polly Alexander (b.abt 1740, d.Aft 1791),
Margaret Alexander (b. Abt.1742 d.Aft 1791),
Hugh Alexander (b. Abt 1747, he participated in the Revolutionary War and many fights with the Indians, much written about him),
Isabel Alexander (b. Abt 1750),
Susannah "Susan" Alexander (1749-1825) who married on 8 Jul 1772 in Mercersburg, PA, John Shannon Alexander (1743-1825)

Lastly, Jacob's first born son was
William Alexander (1734, Down, Ireland - d. btw Nov 1818 - 10 Feb 1819 Great Cove Ayr, Beford, PA)
William was married three times, First wife Unknown and
third/last wife's name was Susannah.
William's second marriage on 14 Nov 1770 to Isabel Alexander (1735-1780,possible cousin of Wm), produced many children, one of which is (id 78211810): Jacob Alexander(1779-1851) (the grandson of this Jacob Alexander)

Some of his siblings included(mentioned in his father's will) - children of William Alexander and Isabel Alexander from above:
Andrew Alexander(b.1786)
William Knox Alexander(1782-1829)
Cairns/Carnes Alexander(1778-1838),
Isabel Alexander(b.1780),
Sarah Ann Alexander(1784-1867),
Susannah Alexander (1784-1831),
John Allen Alexander(1788-1870)
1/2 siblings
Hugh Alexander(b.1767) &
Mary Alexander(1769-1843).

In 1809 Jacob married Susannah Morrison(1791-1861).
They were parents to:
Isabella Alexander(1809-1895),
Esther Alexander (1812-1869),
William Alexander (1812-1875),
Ann Alexander (1813-1893),
David Alexander (1816-1863) &
George Washington Alexander(1819-1902).

In my search for this family's ancestry/history, I have come across some very informative sources/history/info. One of which is a land patent to Jacob signed by President John Quincy Adams. I Googled Jacob Alexander and found there is a book entitled; 'Ohio Genealogical Quarterly' in it is a story/bio entitled, Jacob Alexander And some of his Decendants (pg 461), By William H. Hannah. Also very informative on this family is, THE HISTORY OF FULTON COUNTY PENNSYLVANIA, www.pa-roots.com/beford/history/historyoffultoncounty.

The above information was taken from the memorial of Jacob Alexander (memorial id: 78211810) , the grandson of this Jacob Alexander

I have put burial unknown at this point. I put that death was in Fulton County, Pennsylvania because of the information above.