Another excellent contrivance for the same purpose is described in a French author on clock-making. It was used in the north of England by an ingenious artist about fifty years ago. This inven tion is as follows : a bar of the same me tal with the rod of the pendulum, and of the same dimensions, is placed against the back-part of the clock-case : front the top of this a part projects, to which the upper part of the pendulum is connected by two tine pliable chains- or silken strings, which just below pass between two plates of brass, whose lower edges will always terminate the length of the pendulum at the upper end. These plates are supported on a pedestal fixed to the back of the case. The bar rests upon an immoveable base at the lower part of the case; and is inserted into a groove, by which means it is always re tained in the same position. From this construction, it is evident that the exten sion or contraction of this bar, and of the rod of the pendulum, will be equal, and in contrary directions. For suppose the rod of the pendulum to be expanded any given quantity by heat ; then, as the lower end of the bar rests upon a fixed point, the bar will be expanded upwards, and raise. he upper end of the pendulum just as much as its length was increased ; and hence its length below the plates will be the same as before. Of this pen dulum, somewhat improved by Mr. Crosthwaite, watch and clockmakcr, Dub lin, we have the following description, " A and B (fig. 12), are two rods of steel forged out of the same bar, at the same time, of the same temper, and in every respect similar. On the top of B is form ed a gibbet C ; this rod is firmly support ed by a steel bracket D, fixed on a large piece of marble E, firmly set into the wall F, and having liberty to move freely up wards between cross staples ofbrass, 1, 2, 3, 4, which touch only in a point in front and rear (the staples having been care fully formed for that purpose) ; to the other rod is firmly fixed by its centre the lens G, of twenty-four pounds weight, although it should in strictness be a little below it. This pendulum is suspended
by a short steel spring on the gibbet at C : all which is entirely independent of the clock. To the back of the clock plate, I, are firmly screwed two cheeks nearly cycloidal at K, exactly in a line with the centre of the verge L. The maintaining power is applied by a cylin drical steel-stud, in the usual way of re gulators at M. Now, it is very evident, that any expansion or contraction that takes place in either of these exactly similar rods, is instantly counteracted by the other; whereas in all compensation pendulums composed of different mate rials, however just the calculation may seem to be, that can never be the case as not only different metals, but also die ferent bars of the same metal, that are not manufactured at the same time, and exactly hi the same manner, are found by a good. pyrometer to differ materially in their degrees of expansion and contrac tion, a very small change affecting one and not the other." The expansion or eutitraction of straight-grained fir-wood lengthwise, by change of temperature, is so small, that it is found to make very good pendulum rods. The wood called sapadillo is said to be still better. There is good reason to believe, that the pre vious baking, varnishing., gilding, or soaking of these woods in any n;ielted matter, only tends to impair the property that renders them valuable. They should be simply rubbed on the outside with wax and a cloth. In pendulums of this construction the error is greatly diminish ed, but not taken away.