MESSRS. EDITORS:—In your issue of this week, I find a report, by a committee of the American Engineers' Association, of your city, on my “Low Water Detcctor,” which may leave an unfavorable impression upon the public mind if not noticed. The committee report:—"'Ve examined this instrument at three different places: at the Astor House, where wc learned from the engineer that, in his Dresence and that of our vice-president, the alarm had given warning with two cocks of solid water in the boiler; at the Bible House, where the same thing occurred; and at the Coopet Union, where we foun,1 that the metal had commcnced to corrode after but little use. In view of these facts, your committee cannot recommend it." Allow me, Messrs. Editors, to state what the low water indicator is and how it operates. This instrument is composed of eight feet of common gas pipe, of one ;"nch diameter, having on its upper end a hollow cast iron globe of six inches diameter, and a small chamber beneath the globe to hold the disk or fnsible alloy. The disk is one inch diameter and ene-quarter of an inch thick, and is composed of bismuth, 5 purts ; tin, 3 purts; and lead, 2 parts: its melting point is 212. The detector, when attached to a boiler, has two feet of pipe inside and six outside; tho pipe insipe shonld be within two inches of the flues of the boiler. The operation is as follows: after the boiler has been filled to the water line, and put in action, the pressure of the steam forccs thc water np the pipe into the ail chamber. After a lapse of ten hours, the water fills every part of the instrument. Therc being no circulation of water in the pipe so long as the lower end of the pipe is submerged, the disk will remain solid, as the watcr iu the uppcr part of the instrument is comparatively cool, never rising above 140 (unless the space above the boiler is conYcrted into an ovcn chamber) ; but when the water in the boiler falls to or below the end of the pipe, the water in the pipe falls out by its own gravitation, and the steam cntering, at once melts the disk and sounds an alarm. Since August 1, 1857, I have manufactured, sold and received payment for over one hundred thonsand dollars worth of the above.described low water detector. At this present time, over three thousand of these instruments are in use, giving the highest satisfaction to engineers and owners. It would be difficult te tell how many lives nnd how much property these detectot's bav g saved; but we know of hundreds of instances of their giving timely warning of low water. The committee report ngainst this instrument for two reasons: First, It gnve an alarm nt the Astor House boiler “with two cocks of solid water"—the same at the Bible House ; second, “ At Cooper Union, where we fOllnd the metal had commenced to corrode after but little use." Let us examine this e"idence, and see how it agrees with science and prnctical experience: First, A thousand instnnces exist I"here the disk hns remaine,d at a distance of six feet above the boiler—the distance of. the one on the Astor House boiler—solid for one year. Why should it not be so ! There being no circulation in the instrument unless the disk is not firmly screwed to its seat, the temperatUl'e cnnnot rise above 140, and this heat will not affect the solidity of the disk; but if the disk is not screwed vip tight, and is allowed to drip, then a circulation will commence, and as soon as the temperature of the water in the chamber reaches 212, the disk must melt. A fnlse alarm maybe DJ'oduced by two canses ' one carelessness, the otller ignorance. 1st. If the disk is not made tight to its seat, a cit'culation of water will melt it,- 2d. Cuttinl the pipa too short, bringing it too near the boilar, the Tadiation of heat from which would fuse it. One of two things must be present in the instance nt the Astor House, viz.; a leak in the instrument or a radintion of lleat from the boiler, creating an oven-like heat above the boiler as great as 200. Evidence can be prodnced to show that the alarm at the Astor House was given after a new plug had been placed in the instrament the fim one having l'emained in the detector months before it melted, and then low water was the cause; and since this time tha instrument has not been in favor. If it had had “fair play,.”a new fire box to the bOiler need not have been necessary to the safety of those o ne above the boilers. Second, The corrosion of the plug. Dr. Jackson, Massachusetts State Assayer, says:—.”In reply to your question, whether or not lour safcty plug, consisting of tin, 3 parts; lead, 2 parts ; and bismuth, 5 parts, is liable to be corroded by water when used as directed by you, I would say tkat t4e Hoy is not liable to rapid oxydation from the action of water, and that only a thin film of oxyd will form on the surface of the plug, wbich WIll not impair tts efficiency as a safety plug. It is a small matter to nenew the plug from time to time, say once in four or six months." In yiew ot the facts here stated, I respectfully submit that justice to the low water detector would seem to l'equire a re-consideration at the hands of the committee. E. H. ASHCROFT. Bostan, Mass., Dcc. 7, 18GO. m ?' —