hat worthless mechanisms often got as much space
as sound ones in patent journals, and objections such as the one above
were infrequent. The slanted information thus conveyed to the young
mechanician, who was just accumulating his first kinematic repertory,
was at times sadly misleading.

From even this sketchy outline of the literature on the subject, it
should be fairly evident that there has been available to the
mechanician an enormous quantity of information about mechanical
linkages and other devices. Whatever one may think of the quality of the
literature, it has undoubtedly had influence not only in supplying
designers with information but in forming a tradition of how one ought
to supply the background that will enable the mind to assemble and
synthesize the necessary mechanism for a given purpose.[105]

[Footnote 105: Some additional catalogs of "mechanical movements" are
listed in the selected references at the end of this paper.]

Some of the mechanisms that have been given names--such as the Watt
straight-line linkage and the Geneva stop--have appeared in textbook
after textbook. Their only excuse for being seems to be that the authors
must include them or risk censure by colleagues. Such mechanisms are
more interesting to a reader, certainly, when he has some idea of what
the name has to do with the mechanism, and who originated it. One such
mechanism is the drag link.

After I had learned of the drag link (as most American engineering
students do), I wondered for awhile, and eventually despaired of making
any sense out of the term. What, I wanted to know, was being dragged?
Recently, in Nicholson's _Operative Mechanic and British Machinist_
(1826), I ran across the sketch reproduced here as figure 38. This
figure, explained Mr. Nicholson (in vol. 1, p. 32) "represents the
coupling link used by Messrs. Boulton and Watt in their portable steam
engines. A, a strong iron pin, projecting from one of the arms of the
fly-wheel B; D, a crank connected with the shaft C; and E, a link to
couple the pin A and the crank D together, so the motion may be
communicated to the shaft C." So the drag link was actually a link of a
coupling. Nothing could be more logical. A drag link mechanism now makes
sense to me.

[Illustration: Figure 38.--Drag link coupling used on Boulton and Watt
portable engines. The link E drags one shaft when the other turns. From
John Nicholson, _The Operative Mechanic, and British Machinist_