 Complete the following phrase:  In addition to the ships of the Tisiphone class, the Admiralty expanded its fleet with the acquisition in 1794 of a number of merchant vessels. Most served as small gunvessels, and none were expended as fireships. Another twenty-two small vessels were purchased in 1804, though only four were expended in this role. About this time the Admiralty ordered a six-ship class of fireships. The ships of the resulting Thais class were employed in the sloop role, and were re-rated as such in 1808. They eventually became sixth rate frigates in 1817. Another twenty-one ships were acquired in 1809 for the 