1837.] 



Plan for Metallic-tube Barometers, 



31 



is now poured into this, to stand at the commencement of the scale, or 

 31 inches. The stopcock being- opened puts the instrument in action. 



The brass tube surrounding the glass one, forming the short limb, 

 will be a great security to the latter. Nevertheless it would be an 

 advantage to have one or two to apply in the event of accidents. 



Fig. 3 is the above form of barometer converted into a self-regis- 

 tering one. The shape of the metallic tube, it will be observed, is the 

 same as in the former one. In place of the glass or short limb, there 

 are two rods A, only one of which is seen. The diameter of the upper 

 portion of the long limb C is two inches — that of B half an inch. J 7 

 this latter is a float, so constructed that the part immersed in the nier- 

 cury is of the same specific gravity with that metal, while the part 

 above the surface is as light as possible. The object of this is that 

 when the barometrical column is falling, the light portion of the float 

 will be pushed, as it were, before the mercury ; while during descent 

 the heavy portion will be carried along with that fluid. The pencil D 

 is carried on the top of the rod being pressed gently against the regis- 

 ter roller E, which, corresponding with X in the balance self-register- 

 ing barometer, need not be again described. One of the pencil guides 

 is graduated, and the pencil rod furnished with a vernier, so that, the 

 height of the mercurial column being thus ascertained, the pencil 

 is set at zero— -as already explained. 



V '.—Explanation of the Sketch of a New Indestructible Portable Moun- 

 tain Barometer. — By Captain George Underwood, of the Madras 

 Engineers. 



The tube represented in the sketch is intended to be precisely the 

 same as the glass one used in Gay Lussac's, but constructed of iron, 

 and of the dimensions attached. The projections below marked a a, 

 indicate an inverted cone in the tube, to prevent air rising to the 

 vacuum, and also to check the motion of the mercury while travelling. 

 A thin rod of ir*?n, with tight stoppers at each end, is inserted in the 

 short tube while moving, and a brass cap fixed on the top effectually 

 prevents the loss of any mercury. On the surface of the mercury in 

 the short leg, floats a very light wafer of cork, to which is attached a 

 straight upright wire, with an indicator at the upper end, moving along 

 a scale, each division of which is equal to 4-5th of an inch, the cistern 

 at the end of the long tube being four times the capacity of the short 

 leg. The wire moves through a small hole in a brass cap, screwed 011 

 the upper end of the short tube, and which hole will also serve to give 



