It was Foucault who, towards the end of his life, thought of the immense use of an instrument of this kind as a substitute for the motion of equatorials; he, however, unfortunately did not live to see his ideas realized, but the Commission for the purpose of carrying out the publication of the works of Foucault directed Mr. Eichens to construct a siderostat, and this one was presented to the Academy of Science on December 13th, 1869, and is now at the Paris Observatory. Since that date others have been produced, and they have every chance of coming largely into use, especially in physical astronomy. Fig. 156 shows the elevation of the instrument, the mirror of which, in the case of the instrument at Paris, is thirty centimetres in diameter, and is supported by a horizontal axis upon two uprights, which are capable of revolving freely upon their base. The back of the mounting of the mirror has an extension in the form of a rod at right angles to it, by which it is connected with the clock, which moves the mirror through the medium of a fork jointed at the bottom of the polar axis. The length of the fork is exactly equal to the distance from the horizontal axis of the mirror to the axis of the joint of the fork to the polar axis, and the direction of the line joining these two points is the direction in which the reflected ray is required to proceed. The fork is moved on its joint to such a position that its axis points to the object to be viewed, and, being carried by a polar axis, it remains pointing to that object as long as the clock drives it, in the same manner as a telescope would do on the same mounting. Then, since the distance from the axis of the mirror to the joint of the fork is equal to the distance from the latter point to the axis of its joint to the sliding tube on the directing rod, an isosceles triangle is formed having the directing rod at its base; the angles at the base are therefore equal to each other.