HELIOSTATS, or HELTOSTATA, from ',Aro; the sun, and ra-r41.4,1 I stand, is the name of a very ingenious and useful instrument, invented by Dr s'Gravesande, who was professor of mathematics at Leyden. The object of it is to give such a motion, by means of clockwork, to a polish ed mirror, that the beam of the sun which it reflects may be fixed, or remain in the same position during the diur nal- motion of the earth. Hence it is of great use in all optical experiments, in which it is required to transmit the solar rays into a dark room.
The heliostate, which is represented in Plate CCXC. Fig. 1. consists of two principal parts; lst, A plane metal lic speculum, supported by a stand ; and 2d, A clock for giving motion to the speculum. The speculum S, con tained in a wooden frame, is placed in the brass case a, a, and is suspended by the handle AA, so as to have a free motion round an axis a a, which should pass along the surface of the speculum. The handle AA is joined to the cylinder C, whose axis is coincident with the middle part of the axis a a of the speculum. The axis of the tail DF, joined perpendicularly to the back of the specu lum, is directed to the same point. This tail is made of a cylindrical brass wire, about the sixth part of an inch in diameter. The cylinder C is put .upon the 'wooden stand P, the upper part of which is shewn separately in Fig. 2. The smooth iron cylinder e goes into a cavity in the copper cylinder C, so that, by the motion-of the tail DE, the position of the speculum may be altered at plea sure. The wooden stand P is raised and depressed by the three brass screws B. B, B, turned with a key, and moving in a plate of brass.
The clock is represented at H, with an index which re volves once in 24 hours. The plat.e of the clock is in clined to the horizon, at an angle equal to the co-latitude of the place. The copper pillar FG, which supports the clock, consists of two parts, joined by the screws d, d, between which, as in a sheath, is moved an iron plate, in middle of which there is a slit through which these screws pass. This plate is fixed to the lower plate of the
clock itself, so that it can be raised and depressed, and fastened at any height by the set ews d, d. The same ef fect may also be produced by the screws I, I, I, passing through the copper plates L, L, M. The extremities b c, c' must be in the .same straight line, and a vertical plane passing through them must be perpendicular to the hori zontal lines drawn on the plane of the clock, such asfg, hi.
When the plane of the clock is inclined at an angle equal to the co-latitude of the place, the plane LLM must be brought into a horizontal position by the plummet Q, the point of which is made to coincide with a point o mark ed on the foot M. But if it were required to use the in strument in another latitude, another point o, would require to be marked ; and it would be necessary to incline the plane LLM to the horizon. The axis of the wheel which moves the index has a cylindrical perforation, but a little narrower below than above. The index, shewn separately at ON, in Fig. 3. is made of brass, anti has a tail lig ex actly filling the above mentioned perforation, into which it is thrust tight, so as to stick and be carried along by the wheel. The 'tail p g has also a cylindrical hole, through which passes'the small brass wire l k, which remains in any position into which it is put. At the end o of the in dex there is a small cylinder n perforated cylindrically, The length of the index it n' was six inches in s'Graves ande's machine.
The iron tail t (Fig. 4.) of the piece T goes into the cavity n, Fig..3. as shewn in Fig. 1. and moves in it freely, though not loosely. The small pipe R, Fig. 5. through which the tail DE of the speculum may be moved freely, may be suspended at different heights between the legs of the piece T (Fig. 4.) by the screw r r going into the parts 711 m of the pipe. The pipe can then turn freely round nzm as an axis.