357 



SCREEN. 



SCREW. 



38 



passed by authority of the church, its supporters denying that they 

 required the interposition of any lay legislation. By that measure, the 

 simple objection of a certain number of the male communicants 

 without any reason given, was a cause of disqualification to a pre- 

 sentee. By Lord Aberdeen's Act, any members of the congregation 

 may object to the presentee, stating their objections : and the church 

 courts, if they think them good, whether in their general tenor, or 

 with respect to the particular circumstances of the charge, may give 

 effect to them by rejecting the presentee. 



SCREEN, in architecture, a partition dividing off some portion of an 

 interior or room from the rest, without similarly contracting or shut- 

 ting up the space over head ; a screen being a partition carried up only 

 to a certain height, so as to admit a view beyond it. Screens are ex- 

 ceedingly beautiful internal features in the Pointed Gothic style, in 

 which they were employed for a variety of purposes, not in churches 

 alone, but in halls and other buildings. 



In our larger churches the chancel was separated from the nave by 

 a screen on which was placed the rood. [RooD LOFT.] From the use 

 to which it was subsequently applied this screen is, in our cathedrals, 

 commonly known as the orijan screen ; it differs from others in being a 

 double screen, so as to form a gallery above, and to admit of stairs 

 leading up to it, in the space between two partitions. 



The altar screen serves as a back wall to the choir, separating that 

 division of the church from the presbytery or Lady-chapel behind it. 

 [RETABLE.] That erected by Bishop Fox" in Winchester cathedral is 

 a splendid stone screen decorated with several tiers of canopied niches ; 

 and strikingly similar to it in design is the one by Abbot Wheteham- 

 stede at St. Alban's. Though not so designated, the stalk, &c., form 

 lateral screens enclosing the lower part of the choir from the side 

 aisles. Chartres cathedral contains a no less remarkable than fine 

 example of such screen continued round the apsis of the choir, showing 

 itself as a wall carried up to some height above the stalls, and divided 

 into large compartments filled with sculpture. The fronts of chantries, 

 small chapels, &.<., in churches, may also be described as screens, most 

 of which are pierced or open-work and tracery. The examples of this 

 .re so numerous, that to particularise any of them would be 

 almost superfluous; we may, however, mention that enclosing the 

 monumental chapel of Prince Arthur, son of Henry VIII., in Wor- 

 cester cathedral. The tomb of Henry VII., in his chapel at West- 

 minster, is a gorgeous piece of screen-work, executed entirely in metal, 

 and forming an insulated shrine on a very large scale. Westminster 

 Abbey itcelf contains many fine studies of screens in its chapels and 

 chantries. Of timber screens separating the chancel and altar end 

 from the body of the building, many specimens are to be met with in 

 country churches, and not a few of them are worthy of being studied 

 for the beauty of their design. 



Screens of a different character were employed in the halls of 

 domestic and collegiate buildings, for the purpose of cutting off a 

 passage leading to the butteries and offices. Such screens were almost 

 invariably of oak or other wood, and the space over them and the 

 passage behind nerved as a music gallery. Open-work was rarely if 

 ever introduced into them, but they had generally two open arches, or 

 sometime* square-headed doorways. Several specimens of this class of 

 screens may be seen in Nash's ' Mansions of England in the Olden 

 Times,' some of them, as that in the Hall at Audley-end, profusely 

 adorned with carved panelling and other sculpture ; and the one just 

 mentioned is further remarkable for the centre compartment being 

 carried up higher than the rest, though not quite to the ceiling, 

 instances occur where the screen is carried up two stories, so as 

 to form either a passage on the chamber floor, or a closed gallery with 

 glazed or latticed apertures. Of this kind are those in the halls at 

 Knowle and at Hatfield. 



Screen is also employed to signify a colonnade or wall architecturally 

 decorated, enclosing a court-yard in front of a building, as that for 

 instance of the Admiralty, London. Screens of this kind are some- 

 times had recourse to in order to connect the advancing parts of a 

 plan together, and prevent a facade being cut up into gaps. It is by 

 this means that the buildings forming the river front of Somerset 

 House have been connected together into a continuous facade by 

 screens assuming the appearance of open Corinthian loggias above 

 spacious bridge-like arches. Eminently picturesque in themselves, 

 those features serve to relieve all the rest, and to prevent the monotony 

 that would otherwise take place in so extended a front. 



SCUEW. Thin mechanical power generally consists of two parts, 

 one of which is a solid cylinder of wood or metal, on whose convex 

 surface u formed a projecting rib or fillet, frequently called a thread, 

 which passes spirally round in such a manner as constantly to make 

 equal angles with lines parallel to the axis of the cylinder. The other 

 is a cylindrical perforation through a block of some material, the 

 surface of the perforation having on it a spiral groove correspond- 

 ing to the projecting rib or fillet on the solid cylinder. The first 

 of these parte is called a conrtx screw, and the other a evacuee screw, 

 also a male and female screw. 



A just conception of the nature of the line of direction taken by the 

 rib or groove on the surface of the cylinder, may be obtained" by 

 drawing on a rectangular paper, whose breadth A B is equal to the cir- 

 cumference of the cylinder, any number of lines AB, CD, EF, &c., 

 equidistant from each other, and perpendicular to the sides of the 



paper. Then joining the points A and D, c and F, &c., by right lines, 

 and bending the paper on the surface of the cylinder, the lines A D, 

 c F, EH..&C., will, by uniting at their extremities, become the con- 

 tinuous helix or spiral curve-line which the thread assumes. When 

 the two parts are in action, the convex screw, being turned round in 

 the other by a power applied at its surface, moves at the same time 

 rectilinearly in the direction of its axis : occasionally however the 

 convex screw is fixed, and then the other being turned about, it 

 acquires at the same time a like rectilinear motion. In either case, 

 the path described by a point on any thread during the time that the 



Fig. 2. 



, m 



~\ 



screw turns once on its axis, on being developed, becomes equal to A D 

 or o F ; and in the same time a point on the axis moves through a 

 space equal to B D or D F. 



As a mechanical power, the screw possesses the properties of an 

 inclined plane ; for w representing a weight or pressure at one end of 

 a convex screw, whose threads are thereby made to move in the 

 grooves of the concave screw, let that weight be supposed to act in a 

 direction parallel to the axis, and to be uniformly diffused among all 

 the projecting threads which are at one time in the grooves ; also let p 

 be the part of the weight which presses in the direction mM on an 

 elementary portion iin of the side of a groove in the concave screw. 

 Then Ma may be considered as a small inclined plane, making with mM 

 an angle equal to A D B : and if q be a force which applied at M in the 

 direction N M, touching a circle whose plane passes through the screw 

 perpendicularly to the axis, would prevent the convex screw from 

 turning round ; the pressure on M and the counteracting force will 

 be in the same circumstances as the weight of any body on an inclined 

 plane and a sustaining power which acts in a direction parallel to the 

 base of a plane, and, by the resolution of forces, the ratio between the 

 pressure and the force will be as the base of the plane is to its height; 

 that is, as A B to B D. Now an equal force q will be in equilibrio with 

 the pressure p on every other elementary portion of the grooves in the 

 concave screw ; therefore, there being as many forces = q as there are 

 pressures =p, the whole weight w on the screw will be to the whole 

 sustaining force, in the case of equilibrium as A B to B D ; that is, 

 as the circumference of the convex screw is to the distance between 

 the threads when measured in a direction parallel to the axis. 



But the screw, when applied as a mechanical power, is never used in 

 its simple state ; a lever or wheel is always fixed perpendicularly to 

 the axis, and the moving or sustaining power is applied near the outer 

 extremity of the lever, or at the circumference of the wheel. In this 

 last case, the ratio between the moving-power and the resistance is as 

 the distance between the threads of the screw is to the length of that 

 circumference ; and the velocity of a point on the axis is to that of a 

 point on the circumference in the same ratio. The friction of a screw 

 is however very great, and is frequently equal to, at least, the weight 

 supported, for it will prevent that weight from descending when the 

 moving-power is taken away. 



An endless screw consists of two or more spiral fillets or threads on a 

 rod which is capable of being turned on its axis by a power applied to 



