In order to fix the machine, another part, called the placer, must be used. The cylinder and the speculum being removed from the stand P, the brass pillar VX (Fig. 6.) is stuck tight upon e, Fig. 2. The ruler YZ moves round a centre at X, so as to remain in any position in which it is placed. The arm XZ has a peculiar construction and a certain length, but the length of XY is determined at pleasure. The end X y of the ruler, which does not ex tend beyond y, is included between other two, viz. x Z, and are on the opposite side. They are joined at Z, and are kept together by the screws z, z, which pass through a slit in th_'enclosed ruler X y. On this ruler is mark ed the small line v s, whose length is equal to - parts of the length of the index. The arm XZ is equal to the length of the index, reckoning from the centre of motion at X to the end Z, when x coincides with v, where the divisions on v s commence. The line v s, which is une qually divided, serves to determine the length of the arm XZ for different times of the year, by bringing x to the division corresponding to the given day when the ma chine is used.
In order to mark the divisions, the arm is supposed to be divided into 1000 parts, hence v s will be 90 parts. The distances corresponding to the different times of the year are set down in the following table.
The instrument being thus constructed, is used in the following manner. Having adjusted the point .r of the placer to the division for the day of the month, place XV upon the stand P, (Plate CCXC. Fig. 1.) and put the ruler Y into the position and direction in which it is wished to fix the solar ray. The lines be, b`cf arc placed in a me ridian line drawn on the plane, and the point of the plummet Q is brought to o. The index NO is then turned till the sun's rays pass directly through the pipe R, which may be varied in its position till this is effected. The brass wire I k is then raised or depressed, till the shadow of the end of it / passes through the middle of the pipe R. This
part of the machine is then moved to the placer as formerly adjusted; but the clock is moved and raised in such a manner that the end / of the brass wire 1k may coincide with the end Z of the ruler YZ. The sun's rays, and the shadow of I passing through R, and the point of the plum met corresponding with o; the pillar VX with its ruler is removed, and the cylinder C, carrying the speculum S, is substituted in its plac'e, without altering the position of the stand P. The, piece T is taken out of its place, that the tail DE of the speculum may be put through the pipe R ; and when the piece T is replaced, the machine is then ready for use. The rays reflected from the speculum have the same position and direction as the ruler of the placer ; and while the tail of the speculum moves with the clock, whose index follows the sun, its situation is al tered with respect to the sun, but the ray reflected from the centre of the speculum remains fixed. By substitut ing the index K, Fig. 7. in place of NO, the machine becomes a common clock. For a demonstration of the truth of the preceding construction, the reader is referred to s'Gravesande's Physices Elenzenta Mathenzatica, torn ii. p. 714.
A very simple substitute for the heliostate, as proposed by Dr Thomas Young, is Miami in Fig. 8. A mirror C is fixed on the outside of a window facing the south, op posite the aperture D, through which it is required to transmit the solar rays. Another mirror is., fixed by a joint to a rod AB, moveable about its axis, and parallel to the axis of the earth. The mirror A being placed in a position required for the day, so that the light which it reflects is received at such an angle upon C, that it may be transmitted through the aperture. In order to keep the ray in this position, nothing more is necesssry than to turn the axis BA round on its socket. See Young's Xat. Phil. vol. i. p. 426,785. Analogous contrivances by Pey rard will be found in the Phil. Magazine, vol. xxxvii. p. 183, Sic.