In the organ pipe - as in the common whistle - a thin sheet of air is forced through a narrow slit at the bottom of the embouchure and impinges against the top edge, which is made very p c. sharp. The disturbance made at the commencement of the blowing will no doubt set the air in the pipe vibrating in its own natural period, just as any irregular air disturbance will set a suspended body swinging in its natural period, but we are to consider how the vibration is maintained when once set going.
Outer Seistan, the country on the right bank of the Helmund, and east of its embouchure in the Hamun, extends more than ioo m.
Of the embouchure of the Ribble into the Irish Sea, 182 m.
The only other tributary of any size that the sea receives is the Jaxartes (Sihun, Syr-darya) which enters towards the northern extremity of the east coast, and is suspected to be shifting its embouchure more and more to the north.
Its extreme length is about ioo and its breadth varies from 70 to over ioo m., but the exact limits are vague, and the modern signification of the name practically comprehends the peninsula formed by the lower Helmund and its embouchure on the one side and the Hamun (lake) on the other.