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makers of scientific instruments. It will suffice if we describe two forms, one of which (that of Buddicom) can be made by a laboratory attendant of average intelligence.

In R. A. Buddicom's gas regulator (fig. 20) the inlet (I) and outlet (O) gas pipe open into a metal bell (B), the lower and open end of which is immersed beneath water contained in a metal tray (T). The bell is suspended upon the arm of a balance (B) and the other arm is poised by a weight (W). This weight may be made of any convenient material. In the original apparatus a test-tube partially The weight dips into one limb of a

U-shaped glass tube (U), which con filled

with mercury was used.

B tains mercury. Into the other limb

of this tube the gas from the meter

enters through a glass tube (G) which

1. g; B is held in position by a well-fitting W, cork. The internal aperture of the

Q ' L T tube (G) is very oblique, and it rests 5 si just above the level of the mercury

l ee 'f when the instrument is finally

§ § U | "1 0 adjusted. This adjustment IS better E. 5 ' made in the morning when the gas

==~='? pressure in the main is at its lowest. FIG. 20.-Buddicom's Gas just above the internal aperture of Regulator. the tube (G), a lateral tube (L) passes out from the limb of the U and is

connected with the inlet pipe (I) of the bell. “If the gas pressure rises, the bell (B) is raised and the counter-poising weight (W) is proportionately lowered. This forces the mercury up 1n the other limb of the U-tube and consequently diminishes the size of the oblique orifice in the tube (G). Some of the gas is thus cut off and the pressure maintained constant. Should the pressure fall, the reverse processes occur, and more gas passes through the orifice of G and consequently to the burner by the outlet tube (O). Moitessier's regulator (fig. 21) is more complex, and needs more skilled work in its construction. It consists of an outer and closed cylinder (O), which is filled about half-way up w1th a mixture of acid-free glycerin and distilled water in the proportion of two to one respectively. Within the cylinder is a bell (B), the lower and open end of which dips under the glycerin-water mixture. From the top of the bell a vertical rod (R) passes up through an aperture in the cover of the outer cylinder, and

supports the weighted dish (D). The

R inlet (I) and outlet (O) pipes enter the

chamber of the bell above the level of

the glycerin-water mixture. The outlet

tube is a simple one; but the inlet

tube is enlar ed into a relatively ca a


I; N 8 Y P *

C Q cious cylinder (C), and its upper end is =-E;;; - 55 fitted with a cover which is perforated ”€ 3?-I=§ = by an aperture having a smooth surface M ffl' fl §§ :=§ ; and concave form. Into this aperture 0 ' l 7*-*E an accurately fitting ball- or socket-valve -» (V) fits. The ball-valve is supported

F . . by a suspension thread (T) from the

IG. 21.-Moitessier's

Gas Regulator.

roof of the bell (B). The apparatus

should be adjusted in the morning when

the pressure is low, and the dish (D)

should be then so weighted that the full amount of gas passes through. The size of the flame should then be adjusted. Should the pressure increase, the bell (B) is raised and with it the ball-valve (V). The aperture in the cover of the inlet cylinder is consequently reduced and some of the gas cut off. When the pressure falls again, the ball-valve is lowered and more gas passes through. The relative pressure in the inlet and outlet pipes can be read off on the manometer (M) placed on each of these tubes.

Levelling screws allow of the apparatus being horizontally adjusted. The friction engendered by the working of the vertical rod (R) through the aperture in the collar of the cylinder cover is reduced to a minimum by the rod being made to slide upwards or downwards on three vertical knife-edge ridges within the aperture of the collar. Authorities.-Charles A. Cyphers, Incubation and its Natural Laws (1776); ]. H. Barlow, The Art and Method of Hatching and Rearing all Kinds of Domestic Poultry and Game Birds by Steam (London, 1827); and Daily Progress of the Chick in the Egg during Hatching in Steam Apparatus (London, 1824); Walthew, Artijicial Incubation (London, 1824); Vllilliam Bucknell, The Eccaleobin. A Treatise on Artificial Incubation, in 2 parts (published by the author, London, 1839); T. Christy, jun., Hydro-Incubation (London, 1877); L. Vllright, The Book of Poultry (2nd ed. London, 1893); A. Forget, L'Aviculture et l'incubation artificielle (Paris, 1896); ]. H. Sutcliffe, Incubator: and their Management (Upcott Gill, London, 1896); H. H. Stoddard, The New Egg Farm (Orange Judd Co., New York, 1900); Edward Brown, Poultry Keeping as an Industry (5th ed., 1904); F. ]. M. Page, “A Simple Form of Gas Regulator, " Journ. Chem. Soc. i. 24 (London, 1876); V. Babes, “ Uber einige Apparate zur Bacterienuntersuchung, " Centralblattfur Bacteriologie, iv. (1888); T. HiiPP€, Metlwden der Baeterienforschungen (Berlin, 1889). For further details of bacteriological incubators and accessories see catalogues of Gallenkamp, Baird & Tatlock, Hearson of London, and of the Cambridge Scientific Instrument Company, Cambridge; of P. Lequeux of Paris; and of F. & M. Lautenschliiger of Berlin. That of Lequeux and of the Cambridge Company are particularly useful, as in many instances they give a scientific explanation of the principles upon which the construction of the various pieces of apparatus is based. (G. P. M.)

INCUBUS (a Late Latin form of the classical ineubo, nightmare, from incubare, to lie upon, weigh down, brood), the name given in the middle ages to a male demon which was supposed to haunt women in their sleep, and to whose visits the birth of witches and demons was attributed. The female counterparts of these demons were called succubae. The word is also applied generally to an oppressive thing or person. INCUMBENT (from Lat. incumbere, to lean, lie upon), a general term for the holder (rector, vicar, curate in charge) of an ecclesiastical benefrce (see BENEFICE). In Scotland the title is generally confined to clergy of the Episcopal Church. The word in this application is peculiar to English. Du Cange (Glossarium, s.v. “ Incumbens ”) says that the Jurisconsulti use incumbere in the sense of obtinere, possidere, but the sense may be transferred from the general one of that which rests or is laid on one as a duty which' is also found in post-classical Latin; to 'be “ diligently resident ” in a parish or benetice, has also been suggested as the source of the meaning.

INCUNABULA, a Latin neuter-plural meaning “ swaddling clothes, ” a “cradle, ” “ birthplace, ” and so the beginning of anything, now curiously specialized to denote books printed in the 15th century. Its use in this sense may have originated with the title of the first separately published' list of Isth-CéI'1IllI'y books, Cornelius a Beughem's Incunabula typographiae (Amsterdam, 1688). The word is generally recognized all over Europe and has produced vernacular forms such as the French incurables, German Inkunabeln (Wiegendrucke), Italian incunaboli, though the anglicized incurables is not yet fully accepted. If its original meaning had been regarded the application of the' word would have been confined to books printed before a much earlier date, such as 147 5, or to the first few printed in any country or town. By the end of the 15th century book-production in the great centres of the trade, such as Venice, Lyons, Paris and Cologne, had already lost much of its primitive character, and in many countries there is no natural halting-place between 1490 and 1520 or later. The attractions of a round date have prevailed, however, over these considerations, and the year 1500 is taken as a halting place, or more often a. terminus, in all the chief works devoted to the registration and description of early printed books. The most important of these are (i.) Panzer's Annales typographic ab artis inventae origine ad annum MD., printed in five volumes at Nuremberg in 1793 and subsequently in ISGS carried on to 1 536 by six additional volumes; (ii.) Hain's 'Repertorium bibliographic um in quo libri omnes ab arte typographic inventa usque ad annum MD. typis expressi ordine alphabetic vel simpliciter enumerantur vel adcuratius, recensentur (Stuttgart, 1826-1838). In Panzer's Annales the first principle of division is that of the 'alphabetical order of the Latin names of towns in which incunabula were printed, the books being arranged under the towns by the years of publication. In Hain's Repertorium the books are arranged under their authors' names, and it was only in 1891 that an index of printers was added by Dr Konrad Burger. In 1898 Robert Proctor published an Index to the Early Printed Books in the British M useum: from the invention' of printing to the year MD., with notes of those in the Bodleian Library. In this work the books were arranged as far as possible chronologically under their printers, the printers chronologically under the towns in which they worked, and the towns and countries chronologically in the order in which printing was introduced into them, the total number of books registered being nearly ten thousand. Between 1898 and 1902 Dr W. Copinger published a Supplement to Hain's Repertorium, described as a collection towards a new edition of that work, adding some seven thousand new entries to the sixteen thousand editions enumerated by Hain. From the total of about twenty-three thousand incunabula thus registered considerable deductions must be

made for duplicate entries and undated editions which probably