--------------------------------------------------------------
ft. in. | ft. in. | ft. in. | ft. in.
3 0 | 0 9 | 0 9 | 1 6
3 6 | 0 101/2 | 0 101/2 | 1 9
4 0 | 1 0 | 1 0 | 2 0
4 6 | 1 11/2 | 1 11/2 | 2 3
5 0 | 1 3 | 1 3 | 2 6
6 0 | 1 6 | 1 6 | 3 0
| | |
--------------------------------------------------------------

For higher pressures the blades should be longer and narrower, and
the inlet openings smaller. The case is to be made in the form of an
arithmetical spiral widening, the space between the case and the blades
radially from the origin to the opening for discharge, and the upper
edge of the opening should be level with the lower side of the sweep of
the fan blade, somewhat as shown in Fig. 5.

[Illustration: FIG. 5]

A considerable number of patents has been taken out for improvements
in the construction of fans, but they all, or nearly all, relate to
modifications in the form of the case and of the blades. So far,
however, as is known, it appears that, while these things do exert a
marked influence on the noise made by a fan, and modify in some degree
the efficiency of the machine, that this last depends very much more on
the proportions adopted than on the shapes--so long as easy curves
are used and sharp angles avoided. In the case of fans running at low
speeds, it matters very little whether the curves are present or not;
but at high speeds the case is different.--_The Engineer_.