1.  Jordan R. Dodd, ed.  1993.  Kentucky Marriages:  Early to 1850.  Liahona Research, Orem, UT (Broderbund CD-229):

Downin, Josiah

Mann, Rebecca

27 Sep 1823

Christian Co.

2. 1830 Greene Co., IL, Federal Census transcribed by John C. Jacoby.[link died] Microfilm Roll #M19-24, enumerated 27 Nov 1830 (online at USGenWeb Archives Census Project):

Year	Page #	Line #	Last Name	First Name	Data
1830	42	1	Downing	Joseph	010 010 000 - 300 010 000

These data indicate:

No. & Sex	Age Class	Therefore Born	Individuals Inferred
1 male	5-9	1820-1825	= William (b. 1824)
1 male	20-29	1800-1810	= Josiah (b. 1801/2)
3 females	4 or under	1825-1830	= Mariah (b. 1830) = Elizabeth (b. 1827) = Mary Ann (b. 1825)
1 female	20-29	1800-1810	= Rebecca (b. 1804/5)

Living two doors down from James Mann.  There was no Joseph or Josiah Downen/Downin/Downing in Macon Co.  There is an Alfred M. Downing on p. 47. 

3.  1840 Census Index (online at GenealogyLibrary.com):

1840

Downing, Josiah

IL

De Witt Co.

p. 31

4.  1850 Census Index/Microfilm (online at GenealogyLibrary.com):  T19N R2E, DeWitt Co., IL, p. 431, 485/490, enumerated 12 Dec 1850 (extracted by Diana Gale Matthiesen):

Joseph Downing	48	M	farmer	250	Ky
Rebecca	45	F			Ky
Josiah	16	M	farmer		Ill
Margaret	14	F			Ill
Alfred C.	11	M			Ill
Job	8	M			Ill
Henry C.	6	M			Ill
Josephine	3	F			Ill

5.   LDS.  Ancestral File and IGI (online).  The AF has Josiah Downen married 17 Sep 1823, Christian Co., KY, to "Rebecca Jane TROXEL."  Was MANN her married name?  In the LDS AF, all the children are shown as born in De Witt Co., but De Witt Co. was not formed until 1839 and the census shows them to be in Greene Co., IL, in 1830.  Are these two couples or one?