The attention of owners of steam boilers is yearly becoming more attracted to the utility of high and low water detectors, the latter more especially. We have latterly illustrated and described a number of safety devices of this class, and we this week present to our readers two other inventions of a like character, which have, although recently introduced, acquired considerable reputation as safe and efficient instruments. Fig. 1 is an engraving of a low water detector, and gage cock confbined. The instrument is made of seamless brass tubing, one and' a fourth inches in diameter, and twenty inches in length ; with suitable brass fittings, with gage cock at the lower end, arranged to screw in the boiler in place of the lower gage cock ; and a brass fitting, with steam whistle at the top end. In the center of the tube is a cast iron rod, galvanized to prevent rusting, the upper end holding the whistle valve, and the lower end passing down through the lower casting, which is threaded and arranged for adjusting with a wrench, as may be fully understood by reference to the accompanying engraving. The spiral spring shown in the upper end of the iron rod, is to admit of further contraction of the tube as it cools below 212—the point of adjustment. The operation of this instrument is based upon the different amount of expansion in iron and brass at the same temperature, brass expanding nearly twice as much as iron. The difference in temperature of steam at atmospheric pressure, or fifteen pounds, and at one hundred pounds, is 126". This indicator being carefully adjusted at 212", or boiling water, must be still further expanded when subjected to an increased degree of heat ; and repeated tests have fully proven that twenty pounds of steam is amply sufficient to insure the prompt sounding of the alarm whistle. When the water in the boiler is at the " high water line," or above the "alarm water line," the communication with the detector is submerged, and consequently, the pressure of steam will force water in the detector, the temperature of which cannot exceed 212 Fahrenheit. At this temperature the detector is adjusted, When the water in the boiler descends to the " alarm water line," the water in the detector will gradually descend, by its own gravity, into the boiler, and steam takes its place in the detector ; the increased heat of which will expand the tube nearly twice that of the cast-iron rod, raising the valve seat above the valve, allowing the steam to escape through the whistle, sounding the alarm, which will continue until the water in the boiler rises to a safe hight, when the tube will contract to its former adjustment, and the alarm cease. The extreme simplicity of this device and the sound prin ciples upon which it is based, will be obvious to engineers without further description on our part. A cap not shown in the engraving, screws into the lower part of the instrument, which secures it against being tampered with. Fig. 2 is an illustration of a high and low water detector combined. It is made either of galvanized cast iron or brass, as the purchaser may desire ; both being equally effective, but those of cast iron being of course cheaper.. In the construction of this detector a steam-tight chamber is made, about five inches in diameter by eight or ten inches in hight, with steam whistle at the top, and threaded at the lower end to screw in through a one and a half inch hole on the top of the boiler; to the lower end of this chamber is at- tached a light one-inch brass pipe, extending down to the " alarm water line," or say one or two inches above the flues. Inside of this chamber is a metal float, about four and a half inches in diameter, and seven to nine inches in length, with the whistle valve attached to its top end. To the lower end of this float a light brass pipe, a half inch in diameter, is attached, and extended down even with the outer pipe, or to the "alarm water line." Through thishalf inch pipe a fourth inch pipe is passed to the top of the float, and perforated so as to allow steam to pass out in the interior of the float ; the lower end of this small pipe being curved up on a level with the "high water line," as shown in the engraving. When the water in the boiler is at the " true water line," the pressure of steam will force water up in the chamber through the larger pipe until the float rises and closes the valve. The float, having both steam and water connections with the boiler, when the water is below the "high water line," must be filled with steam which will equalize the pressure and prevent collapsing. Now, it is obvious that when the water in the boiler falls below the " alarm water line," and below the end of the pipe communicating with the chamber, that the waterin the chamber will, by its own gravity, descend into the boiler, and its former space be occupied by steam, and that the float, now becoming a weight, falls to the bottom of the chamber, opening the valve and sounding the alarm. In case the water in the boiler rises above the " high water line," both communications with the float being submerged, it will soon fill with water and fall to the bottom of the chamber, opening the valve, allowing water to escape through the whistle, which must soon attract the attention of the engineer and prevent damage. When steam goes down in the boiler, air will pass in through the whistle, which at once becomes a perfect and certain vacuum valve. This indicator, being automatic, will adjust itself immediately after the water in the boiler falls below the "high water line," or rises above the alarm water line, ten seconds of time only elapsing before it announces low water, and one minute for high water. This instrument is, we believe, the only one yet patented which has pressure equalized on outside and inside of the float, thus insuring it against collapsing or sinking. The first prize, a bronze medal, has just been awarded this instrument, at the recent Massachusetts State Fair, held at Boston. Patents were obtained through the Scientific American Patent Agency for these instruments as follows, by G. B. Mas-sey, of New York : On the first described instrument, Feb. 23, 1869 ; on the second Sept. 28, 1869 ; and patents on the same have been secured in France and England. Address, for further particulars, J. W. Blake Co., 56 John Street, General Agents for the Massey Low Water Detector Company.