ument. The lower portion
of the instrument as formerly made for me by the late Mr. Tisley,
optician, Brompton Road,[28] is a cap that surrounds the body of the
whistle, and is itself fixed to the screw that forms the plug. One
complete turn of the cap increases or diminishes the depth of the whistle,
by an amount equal to the interval between two adjacent threads of the
screw. For mechanical convenience, a screw is used whose pitch is 25 to
the inch; therefore one turn of the cap moves the plug one twenty-fifth
of an inch, or ten two-hundred-and-fiftieths. The edge of the cap is
divided into ten parts, each of which corresponds to the tenth of a
complete turn; and, therefore, to one two-hundred-and-fiftieth of an
inch. Hence in reading off the graduations the tens are shown on the
body of the whistle, and the units are shown on the edge of the cap.

The scale of the instrument having for its unit the two-hundred-and-
fiftieth part of an inch, it follows that the number of vibrations
in the note of the whistle is to be found by dividing (13440 x 250)/4
or 84,000, by the graduations read off on its scale.

A short table is annexed, giving the number of vibrations calculated
by this formula, for different depths, bearing in mind that the
earlier entries cannot be relied upon unless the whistle has a very
minute bore, and consequently a very feeble note.

===================================
| Scale Readings | Corresponding |
| (one division | Number of |
| = 1/250 | Vibrations |
| of an inch). | per Second |
|----------------+----------------|
| 10 | 84,000 |
| 15 | 56,000 |
| 20 | 42,000 |
| 25 | 33,600 |
| 30 | 28,000 |
| 35 | 24,000 |
| 40 | 21,000 |
| 45 | 28,666 |
| 50 | 16,800 |
| 55 | 15,273 |
| 60 | 14,000 |
| 65 | 12,923 |
| 70 | 12,000 |
| 75 | 11,200 |
| 80 | 10,500 |
| 85 | 9,882 |
| 90 | 9,333 |
| 95 | 8,842 |
| 100 | 8,400 |
| 105 | 8,000 |
| 110 | 7,591 |
| 115 | 7,305 |
| 120 | 7,000 |
| 125 | 6,72