air, the temperature of the stove is kept within the required limits; and owing to the slow-conducting power of the fire-clay, of which the fire-box is formed, the heat of the fuel is concentrated within the fire-box, and the fuel burns with less air and less rapidity than it would otherwise do. The construction of Dr. Arnott’s “thermometer stove” will be better understood from the following figure, which represents the stove with one of its sides removed, so as to exhibit its interior arrangements:-- [Illustration] The outlines of the figure, _a a a a_, represent the case or body of the stove, which might be formed either of cast or sheet iron. It is divided into two chambers by the partition, _b b_; but in such a way that there may be a free communication at the top and bottom. _c_ is a small furnace, or, as it is called by the inventor, a fire-box, made of iron, and lined with fire-bricks. The fire-box is not in contact with the exterior case of the stove. It communicates at the bottom with an ash-pit, the door of which is at _d_,--that of the stove, by which the fuel is introduced, is at _d´_. Both these doors must fit very accurately. Above the door of the ash-pit is a bent pipe _e_, by which air gains admittance to the fire. A fire being kindled and the doors at _d d´_ shut, the only way in which air has access to the fuel is by the pipe _e_; the air so admitted, passing through the fire before it enters the upper part of the stove. That portion of the air not required to aid the combustion of the fuel having reached the main body of the stove, and there mixing with the smoke and other products, they circulate slowly in the directions indicated by the arrows, and at length pass into the chimney by the pipe _f_. The slow movement just mentioned as taking place within the stove may well be contrasted with what happens in an open fire-place. In one case the greater part of the heat produced is rapidly carried off by a current of air ascending the chimney--by the thermometer stove it is detained until almost the whole of it has been diffused throughout the apartment. The bent tube _g_ terminating in a cup-shaped opening at _g´_, is a self-regulating valve. The tube is closed at the end _g_ within the stove, _g´ g´´_ represents mercury which occupies the bend of the tube. When the fire in the stove burns too briskly, the air in the tube occupying the space between _g_ and _g´´_ is expanded, and by expelling some of the mercury from the tube at _g´´_ into the cup at _g´_, it closes the aperture of the pipe _e_; thus cutting off the supply of air to the fire. In a few minutes (the fire in the mean time having abated its energy,) the air in the tube will return to its former dimensions, and the mercury subsiding in the cup, air is again permitted to enter the ash-pit.