effective diameter of the material on the bobbin changes with the
beginning of each new layer.

The eyes of the flyers always rotate in the same horizontal plane,
and hence the rove always passes to the bobbins at the same height
from any fixed point. The bobbins, however, are raised gradually by
the builder during the formation of each layer from the top of the
bobbin to the bottom, and lowered gradually by the builder during
the formation of each layer from bottom to top. In other words, the
travel of the builder is represented by the distance between the
inner faces of the flanges of the rove bobbin.

[Illustration: FIG. 19 ROVING FRAME FAIRBAIRN'S ROVING FRAME IN WORK]

Since every complete layer of rove is wound on the bobbin in virtue
of the joint action of the spindle and flyer, the rotating bobbin,
and the builder, each complete traverse of the latter increases the
combined diameter of the rove and bobbin shaft by two diameters of
the rove. It is therefore necessary to impart an intermittent and
variable speed to the bobbin. The mechanism by means of which this
desirable and necessary speed is given to the bobbin constitutes one
of the most elegant groups of mechanical parts which obtains in
textile machinery. Some idea of the intricacy of the mechanism, as
well as its value and importance to the industry, may be gathered
from the fact that a considerable number of textile and mechanical
experts struggled with the problem for years; indeed 50 years
elapsed before an efficient and suitable group of mechanical parts
was evolved for performing the function.

The above group of mechanical parts is known as "the differential