In what is called the _equal temperament_ system the attempt is made to divide the octave into twelve equal parts or semi-tones, thus rendering all keys alike. To do this it is necessary to slightly flatten all the fifths and sharpen the major thirds. The difference from just intonation is about one-fiftieth of a semi-tone. Although recommended and used by J. S. Bach, equal temperament was not introduced into English organs until 1852. Much has been lost by adopting equal temperament, but more has been gained. To a sensitive ear, the sharp thirds and fourths, the flat fifths and other discordant intervals of our modern keyed instrument, are a constant source of pain; but the average organist has become so accustomed to the defect that he actually fails to notice it! The change to equal temperament has on the other hand greatly increased the scope of the organ and has rendered possible the performance of all compositions and transcriptions regardless of key or modulation. The tuning of an organ is seriously affected by the temperature of the surrounding air. Increased heat causes the air in the open pipes to expand and sound sharp contrasted with the stopped pipes through which the air cannot so freely circulate. The reeds are affected differently, the expansion of their tongues by heat causing them to flatten sufficiently to counteract the sharpening named above. Hence the importance of an equable temperature and the free circulation of air through swell-boxes, as described on page 59, _ante_. NEW METHOD OF REED TUNING. Organ reed pipes, especially those of more delicate tone, fail to stand well in tune, especially when the tuner is in a hurry or when he does not know enough of his business to take the spring out of the reed wire after the note has been brought into tune. Few persons fully understand the reason why reeds fail to stand in tune as they ought to. [Illustration: Figs. 31-35. New Method of Tuning Reeds] Figures 31, 32, and 33 will serve to make clear the chief cause for reeds going out of tune. Figure 31 may be taken to represent a reed block, eschallot, tongue and tuning wire at rest. In this case the tuning wire will be pressing firmly against the tongue at the point B, but said tuning wire will not be subjected to any abnormal strain. Turning to Figure 32, if we use the reed knife and slightly lift the tuning wire at the point C, friction against the tongue at the point B will prevent said point B from moving upward. (In this connection it must be borne in mind that the co-efficient of friction in repose is much greater than the co-efficient of friction in motion.) In consequence of the drawing up of the tuning wire at point C, and the frictional resistance at point B holding the latter steady, the lower part of the tuning wire will assume the shape shown in Figure 32, and point A will in consequence move farther away from the tongue.