" McNaiight indicators " were being made at that establishment, and 

 the demand was very light ; but since we have the beautiful inven- 

 tion of Richards, the indicator is a necessity to every engineer, and 

 they are made and sold by the thousands. So would it be with a 

 good registering indicator, were it invented. 



Teanslucent Mirror. 



Dr. Adolph Preterre exhibited a translucent mirror, and distributed 

 specimens among the audience. An ordinary glass plate is coated 

 with a salt of platinum, and placed in an oven heated sufficiently to 

 fuse it with the surface of the glass. The result is, that when an 

 opaque substance is placed behind it, the glass will serve as a reflector 

 or mirror, but without siicli backing the glass will be translucent. 

 It was said that this glass was used in Berlin, Prussia, for windows, 

 which admitted light in the day-time, and served as mirrors when 

 the shutters were closed in the evening. The glass is manufactured 

 by Faure and Dore, in Paris, France. 



Mr. J. K. Fisher said that it has been discovered that all metals, 

 when very thin, are transparent. Eye-glasses have been made on 

 this principle ; they are coated with a thin film of gold or silver, 

 which mellows the light. These eye-glasses are used by the work- 

 men to protect their eyes while at work in making the Bessemer 

 steel. 



Mr. G. II. Babcock remarked that this mirror had only a thin 

 coating of platinum. 



Mr, T. D. Stetson stated that there were many situations in which 

 this method of coating glass would be valuable, as in places where it 

 is desirable to reflect a portion of light and transmit another. For 

 street lamps also, where most of the light should be thrown down, 

 or along a dock. 



Hall's Automatic Detachable Weight for Bathometer. 

 Dr. A. "W. Ilall exhibited his detachable weight for his Bathome- 

 ter. It consists of a strong float balanced by means of an arrange- 

 ment secured to the bottom, so that it will stand in an upright 

 position about half out of water. The float is closed at the top, but 

 there are holes at the bottom to allow the water to enter. If the air 

 is absorbed by the water during compression, its place is supplied 

 again by a small cup at the bottom filled with a very volatile vapor, 

 such as cliloroform, which would give ofi* a gas or vapor. On the 



