The "Pre-Paid" letter was mailed on July 22, 1852 from Oxford, Oxfordshire, England as indicated by the blue Oxford CDS's. It was routed through London where it received the red Paid July 22 CDS, (lower right-hand corner). The cover was then routed from London to Liverpool where it received the L JY 23 A marking in black.

The Royal Mail Packet; Cunard Steamship, "Niagara" departed Liverpool on July 24, 1852 and arrived in Boston on August 5th, where the letter received the "Br. Packet 5 Aug 24 (cents) Boston,", the straight line "PAID" and "5 Cents" markings. There is also a script "Paid 1 Shilling" at bottom left of the cover, which was applied at the point of origin.

Cunard liners and the Niagara
Samuel Cunard won his original contract to carry the mails to North America in 1840. He won a new contract for the Trans-Atlantic Mail on July 1, 1846, inclusive through January 1858 and began work on four new steamships; the America, Canada, Niagara, and Europa. This effectively spelled the end for the American owned Great Western Steamship Line, which no longer had the funds to compete against Cunard. Although popularly known as the Cunard line after it's owner, the official name of the line was "The British and North American Royal Steam Packet Company."

The Niagara was constructed by Robert Steele and Company in 1847. She was propelled by Side Lever Paddlewheels with a gross tonnage of 1834 grt. She was 251 feet in length overall with a beam of 38 feet. She entered service in 1848, departing on her maiden voyage from Liverpool on May 20, 1848. In 1854, Great Britain became embroiled in the Crimean War with Russia and began commandeering Cunard Steam Ships for use as troop carriers. The Niagara was commandeered in November 1854 and was out of service through most of 1855; returning to to the Atlantic mail service on May 24, 1856 and arriving in Boston on June 6th. 

In 1862 Cunard added two new Steamships, the Scotia and China to his fleet. Niagara made her final trans-Atlantic crossing from May 3 to June 8, 1862. She was sold to Duncan Dunbar in 1866 and converted to sail.

NOTES:
1) The Scotia was the most powerful wooden side-wheel steamer ever built.
2) The Queenstown (Cobh) Ireland stopovers did not begin until November 1859.

SEE ALSO:
1858 Stampless Folded Letter per "Asia" from Liverpool to New York 
1866 Printed Circular per "Asia" via "Queenstown" from Liverpool to Boston 
for additional  information on the Cunard Line and Packet Rates.

Thomas Hunt
The most likely T. A. Hunt is Thomas Hunt born in 1828 in England and listed on the 1860 census as living in Smithfield, RI. The letter is addressed from Oxford, England and mentions his mother being in a hurry to write, indicating that she is still living in Oxford. 

Oxford, Oxfordshire, England
The town of Oxford is located approximately 50 miles northwest of London. There is some evidence of settlement in the area as early as 4000 BC, although it didn't become a major town until the time of Alfred the Great, who fortified it against Danish attack. When the Danish King Cannute conquered most of southern England, he chose Oxford for his coronation. Oxford University, believed to have been established by Alfred is located here. By the 13th century, Oxford was firmly established and students from all parts of Europe were gathered here.

The message on the inside of the flap reads as follows:
Dear Friend, - being very busy this week; I have not had time to write. I will endeavor to do so next time, your mother being in a hurry to write, because of the money. F. S.--

Sources and Suggested Additional Reading:
Steam and the North Atlantic Mails (Cunard Line); J. C. Arnell - 1986 
North Atlantic Mail Sailings: 1840 to 1875; Hubbard & Winter - 1986 
U.S. Incoming Steamship Mail 1847 to 1875; Wierenga - 2000 
American Stampless Cover Catalog - Volume II; David G. Phillips Publishing - 1987 
Clyde Built Data Base 
Ancestry.com