George Hogarth, 19th century musicologist and critic
Today I added the below caption to the Alto Trombone Timeline, bringing the total number of primary sources for the timeline to 90. Noteworthy is the fact that the author clearly states that the alto trombone is pitched in F, a fourth above the tenor trombone, not simply in the same key as the tenor with a smaller mouthpiece.
1837—London, England: George Hogarth, Scottish music critic and musicologist, specifies the keys of the members of the trombone family in an article in The Musical World, the preeminent British music journal of the time: “Thus the bass trombone gives the fundamental note G…The tenor trombone is in C, a fourth higher than the bass trombone; and the alto trombone is in F, a fourth above the tenor trombone…” (Hogarth 132).
Kareem Waters
While there is not a great deal of solo repertoire for the trombone from the eighteenth century, there are a few pieces, as well as soloistic parts for the trombonist in a number of larger works. As is the case with first trombone parts in orchestral works, these solo parts are sometimes labeled Altposaune (“alto trombone”), and so the prevailing assumption in recent years has been that an alto trombone as we would know it, pitched a fourth or perhaps a fifth higher than the tenor, is the appropriate instrument for playing these pieces. That assumption has been effectively challenged by Howard Weiner, who in a 2005 Historic Brass Society Journal article suggested that the terms “alto,” “tenor,” and “bass” on trombone parts had more to do with the range and function of the part than the required instrument, and that for the highest part in particular what we would call a small-bore tenor trombone, with a small mouthpiece, was used much more commonly than the alto trombone. Although his remains (for now) a minority opinion, Weiner makes a compelling case, especially when one considers that a number of the trills and other ornaments called for in these works (including in the Albrechtsberger) might be easier to execute (in terms of slide motion at least) on a small tenor trombone.
wkimball
See this blog post for a thorough reply about the whole alto=tenor hypothesis.