According to original Regimental cards filed in the Compiled 
Service Records in the National Archives:

"John Earnest appears on the Muster and Descriptive Roll of a
Detachment of U.S. Vols. forwarded for the 7th Reg't Pennsylvania 
Cavalry."

"Roll dated at Harrisburg, PA  February 29, 1864 (Leap Year!--ed. note)
Born at Columbia, PA, age 20 years (error-22yrs--ed. note).  Occupation at
enlistment was farmer.  Brown hair, brown eyes, dark complexion, 5'10" in
height. Bounty paid $60.00, $240.00 due him. Mustered in at Harrisburg on
Feb. 27, 1864. Enlistment credited to Rush Township, Northumberland County,
PA, 14th Congressional District."

John had enlisted as a Veteran Volunteer, which gave him extra bounty money
(enlistment bonus). The 7th PA Cavalry had been in Tennessee and Kentucky
for 3 years and had hard fighting. In February of 1864, they went on leave
and came home to recruit. John and his brother, William, joined up, were
taken by train from Harrisburg to Pittsburgh, where the ladies of the city
greeted them with a magnificent breakfast which was remarked upon for years
after the war when the old soldiers would get together at reunions. From
Pittsburgh they were shipped on to Nashville....

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                                                        the
Nashville Tenasee                               March   9  1864

we arived at Nashville tenasee this morning Safe and Sound  We had bin
trailing for five days and knights  ihad expected to git a furlough but it
was in vain  iSent Sixty dolars to catawissa and igive the revernt Mr P
tarence to hundard and twenty five to give to marts to give to father  if
he dont buy that land of isril barningers i want him to out it on intrest
for me til icome home ware it is Safe  iwill Sent my Picture home as Soon
as igit it from harisburgh iwant them to git Mahalas Photagraph and Send me
one iam in Company A   7 Pensylvania regt Cavlry  under Capt Wite  William
Earnest is in Company H