[Fig. 1132.] shows a pair of thermostatic bars, bolted fast together at the ends a. The free ends b, c, are of unequal length, so as to act by the cross links d, f, on the stopcock e. The links are jointed to the handle of the turning plug of the cock, on opposite sides of its centre; whereby that plug will be turned round in proportion to the widening of the points b, c. h, g, is the pipe communicating with the stopcock.
Suppose that for certain purposes in pharmacy, dyeing, or any other chemical art, a water-bath is required to be maintained steadily at a temperature of 150° F.: let the combined thermostatic bars, hinged together at e, f, [fig. 1133.], be placed in the bath, between the outer and inner vessels a, b, c, d, being bolted fast to the inner vessel at g; and have their sliding rod k, connected by a link with a lever fixed upon the turning plug of the stopcock i, which introduces cold water from a cistern m, through a pipe m, i, n, into the bottom part of the bath. The length of the link must be so adjusted that the flexure of the bars, when they are at a temperature of 150°, will open the said stopcock, and admit cold water to pass into the bottom of the bath through the pipe i, n, whereby hot water will be displaced at the top of the bath through an open overflow-pipe at q. An oil bath may be regulated on the same plan; the hot oil overflowing from q, into a refrigeratory worm, from which it may be restored to the cistern m. When a water bath is heated by the distribution of a tortuous steam pipe through it, as i, n, o, p, it will be necessary to connect the link of the thermostatic bars with the lever of the turning plug of the steam-cock, or of the throttle valve i, in order that the bars, by their flexure, may shut or open the steam passage more or less, according as the temperature of the water in the bath shall tend more or less to deviate from the pitch to which the apparatus has been adjusted. The water of the condensed steam will pass off from the sloping winding-pipe i, n, o, p, through the sloping orifice p. A saline, acid, or alkaline bath has a boiling temperature proportional to its degree of concentration, and may therefore have its heat regulated by immersing a thermostat in it, and connecting the working part of the instrument with a stopcock i, which will admit water to dilute the bath whenever by evaporation it has become concentrated, and has acquired a higher boiling point. The space for the bath, between the outer and inner pans, should communicate by one pipe with the water-cistern m; and by another pipe, with a safety cistern r, into which the bath may be allowed to overflow during any sudden excess of ebullition.
[Fig. 1136.] is a thermostatic apparatus, composed of three pairs of bars d, d, d, which are represented in a state of flexure by heat; but they become nearly straight and parallel when cold, a, b, c, is a guide rod, fixed at one end by an adjusting screw e, in the strong frame f, e, having deep guide grooves at the sides. f, g, is the working-rod, which moves endways when the bars d, d, d, operate by heat or cold. A square register-plate h, g, may be affixed to the rod f, g, so as to be moved backwards and forwards thereby, according to the variations of temperature; or the rod f, g, may cause the circular turning air-register i, to revolve by rack and wheel-work, or by a chain and pulley. The register-plate h, g, or turning register i, is situated at the ceiling or upper part of the chamber, and serves to let out hot air. k, is a pulley, over which a cord runs to raise or lower a hot-air register l, which may be situated near the floor of the apartment or hothouse, to admit hot air into the room. c, is a milled head, for adjusting the thermostat, by means of the screw at e, in order that it may regulate the temperature to any degree.
[Fig. 1137.] represents a chimney, furnished with a pyrostat a, b, c, acting by the links b, d, e, c, on a damper f, h, g. The more expansible metal is in the present example supposed to be on the outside. The plane of the damper-plate will, in this case, be turned more directly into the passage of the draught through the chimney by increase of temperature.