Some surprise may have been excited by the fact that the well known method of revivifying hydrated calcium sulphate has recently formed the subject of a patent (Eng. pat., No. 15,406).

The method described in the specification consists in reducing the materials (waste moulds, etc.) to small lumps, and baking between the temperatures of 95 deg. and 300 deg..  It is mentioned that the whole of the water must not be expelled.  This is no doubt correct, but it must be effected by regulating the time of baking, since by prolonging the operation all the water of crystallization can be expelled far below 300 deg..  To secure even baking the mass is kept stirred by mechanical stirrers, a necessary precaution, since the operation is to be carried out in an ordinary kiln.  The process is stopped when a portion of the plaster is found to set in the required time, a method of regulation which will probably be found to work well in practice.—­Chem.  Trade Jour.

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SPACING THE FRETS ON A BANJO NECK.

BY PROF.  C.W.  MACCORD.

The amateur performer on the banjo, if he be of a mechanical turn, is often tempted to exercise his skill by making an instrument for himself; and the temptation is the greater because he can confine himself to the essentials.  The excellence of a banjo in respect to power and tone depends mainly upon the rim and the neck, that is, supposing the parchment head to be of proper quality; but then the preparation of the heads is a business of itself, and the amateur is no more expected to make the head than to make the strings.  So again, all the minor accessories, such as pegs and tail pieces, brackets and bridges, are kept in stock for his benefit, and he may justly claim all the credit if his efforts in connection with the two principal parts first mentioned result in the production of a superior instrument.  Among these ready-made items is a “fret wire” of peculiar section, furnished with a flange ready for insertion into fine saw cuts across the neck, which much facilitates his work.

Of course, the correctness of the notes depends entirely upon the accuracy with which the frets are spaced, and the accompanying diagram exhibits a convenient method of determining the spaces by graphic means.

[Illustration:  SPACING FOR BANJO FRETS]

It is to be understood that when the distance from the “nut,” N, to the bridge, B, has been determined, the first fret is to be placed at 1/18 of that distance from the nut, the distance from the first to the second is to be 1/18 of the remainder, and so on.  To determine these distances by computation, then, is a simple enough arithmetical exercise; but it is exceedingly tedious, since the denominators of the fractions involved increase with great rapidity; being successive powers of the comparatively large number 18, they soon become enormous.