18370 



Plan for Metallic-lube Barometers. 



corresponding elevation of the other end of the balance, and of course 

 of the pencil bearing on the register cylinder below. 



The indications on the roller may be magnified several times by 

 making the register arm of the balance longer than the other. 



Metallic-tube Barometers. — The very great liability of the com- 

 mon barometer to derangement from the entrance of air, and the 

 great danger of destroying the instrument in attempting to expel this 

 by the only efficient mode—viz. boiling the mercury in the tube — has 

 long been the bitter complaint of the scientific traveller and meteoro- 

 logist. With reference to India, the truth of this remark is abundant- 

 ly evident. I have met with very few who possess barometers, the 

 uselessness of which, simply from air having entered the tube, did not 

 occasion them to regret their distance from the instrument maker. 

 Various plans have been invented to prevent, as much as possible, this 

 liability to the entrance of air ; amongst which perhaps the best is the 

 walking-stick barometer of Englefield : but all have the common dis- 

 advantage of glass tubes, and therefore are subject to be destroyed by 

 fracture when the mercury is attempted to be boiled in them. I con- 

 sider it practicable to substitute iron for glass, in so much of the baro- 

 meter as it is necessary to expose to heat to expel air, and consider 

 that the plan, represented by fig. 2, of the accompanying sketches, will 

 fulfil that desirable object. ABC represents a tube of malleable iron, 

 which is forged in a straight line, and bent as represented after a hole 

 has been bored through it. The short limb C D consists of a com- 

 mon barometer glass tube, encased in one of brass, the intervening 

 space being filled up with magnesian cement. The lower part of the 

 glass tube passes through a disc of iron, which is convex below to 

 apply exactly to the somewhat hollow expanded end of the metallic 

 tube. On the side of this expanded end is a screw which fits a cor- 

 responding one inside of the lower end of the brass tube ; by which 

 means the two limbs are firmly connected ; also being made to apply 

 exactly, thereby preventing any escape of mercury. In order to ensure 

 the exact adaptation of these two parts, the iron disc and expanded 

 portion of the metallic tube are ground together before the latter is bent. 



The part M, or the upper portion of the tube, has a diameter of one 

 inch — the remaining portion is about f ; less would do, but a smaller 

 hole I fear could not be drilled so long. The advantage of having the 

 top of the tube enlarged is twofold — 1st, a much shorter portion is 

 required to be ground accurate after boring — and 2dly, the indications 

 in the glass or short limb are greater in the inverse proportion of that 

 of the squares of the diameters of the two parts of the instrument. 



