In the pyrometer invented by M. De Saintignon, high temperatures are measured by inserting in the furnace a tube through which a current of water is passing at a uniform rate. The temperature of the water is measured by a mercurial thermometer as it enters the tube, and again as it leaves, and from the difference of the two readings the intensity to which it has been exposed is deduced. The instrument is made in two forms. In the first, it is applicable to heated spaces with thin walls, such as smoke boxes; and in the second, to furnaces enclosed with masonry.
It is the latter form of which we annex an illustration. It comprises two thermometers, T1, T2, graduated with long scales, and connected by elastic tubes to the pipe, E, which is passed through the wall into the furnace the temperature of which it is desired to measure. A uniform current of water flows from a reservoir situated at a height of about ten feet above the pyrometer, passes through a filter, and descends into a vessel encircling the bulb of the thermometer, T1. This thermometer indicates the initial temperature of the water. From it the water flows by the elastic tube, E1, into the copper tube, E, which is situated within the furnace, and is exposed to its heat at the particular point where it is stationed for the time being. The water becomes heated in its passage, and returns to the second thermometer, E2, where its temperature is again measured. The speed of the current, and the length of the tube exposed to the action of the fire, are so adjusted that the water is raised one degree for each twenty-five degrees of the furnace. A plate, P, furnished with a handle, p, carries a scale advancing twenty-five degrees for each degree of the thermometer scale; its zero point is placed opposite the head of the column of mercury in the thermometer, T1, and the temperature is read opposite the top of the mercury in T2. After leaving the second thermometer, the water enters the pressure gauge, M, which consists of a tube open at its upper extremity, and carrying at its lower end a cock, R, by which the water escapes. By adjusting the cock, S, the flow is regulated until the water rises to the mark, B, in the tube; and so long as it does not vary from this point, it is known that the calculated discharge is taking place through the cock, R.
 FR patent No 125,094
 LU patent No 42
 DE patent No 22,223; GB patent No 2,400/1878