Another kind of harmonics must now be described, called combinational or Tartini tones (from Tartini, a celebrated Italian violinist of the XVII century, who first described them). "These tones," says Helmholtz, "are heard whenever two musical tones of different pitches are sounded together loudly and continuously." There is no necessity for giving a table of all of their tones here; we select the two most useful. If two notes at an interval of a fifth are held down, a note one octave below the lower one will be heard. So organ builders take two pipes--one 16 feet long (CCC) and one 10 2/3 feet long (GG)--which make the interval of the fifth, and, by sounding them together, produce the tone of a pipe 33 feet long (CCCC). This is the stop which will be found labeled "32-ft. Resultant." Hope-Jones makes a stop which he calls Gravissima, 64-ft. Resultant, in his large organs. Many contend that this system produces better results than if pipes of the actual lengths of 32 or 64 feet were employed. Indeed, a pipe 64 feet long would be inaudible; the human ear has its limitations and refuses to recognize tone lower than 32 feet (just as we cannot lift water by a suction pump over 32 feet)--_but_, these great pipes _produce harmonics_ which wonderfully reinforce the tone of the organ. Therefore their use is worth while. The other combinational tone to which we refer is that produced by the interval of a major third. It sounds two octaves below the lower note. The writer is not aware that this has ever been used as an organ stop, but it is found written in the organ compositions of Guilmant and other first-rate composers. It will be seen that a skilful organist, with a knowledge of these tones, can produce effects from small organs not available to the ordinary player. Reverting once more to our Flute, whose tube is shortened by lifting the fingers from the holes, it is not generally known that this can be done with an organ pipe; the writer has met with instances of it in England. The two lowest pipes of the Pedal Open Diapason were each made to give two notes by affixing a pneumatic valve near the top of the pipe. When the valve was closed the pipe gave CCC. When the organist played CCC sharp, wind was admitted to the valve, which opened, and this shortened the pipe. The device worked perfectly, only that it was not possible to hold down both CCC and CCC sharp and make "thunder"! The organist of Chester Cathedral had been playing his instrument twice daily for ten years before he found this out, and then he only discovered it when the pipes were taken down to be cleaned. It is an admirable makeshift where a builder is cramped for room. Organ pipes are divided into three families--Flues, Reeds and Diaphones. The flues are subdivided into Diapasons, Flutes, and Strings, and we now proceed to consider each of these groups separately. DIAPASONS.