70 Ccqu. Underwoods Plan for an Indestructable Barometer. [July 



of an indestructable and self-registering barometer, as iron was like- 

 wise mentioned by myself, I trust I may be pardoned for throwing out 

 the following suggestions, although I am not certain that Mr. Taylor 

 commented at all on the plan proposed by me. since " No. 2" merely is 

 mentioned, without names being specified. The approval which that 

 scientific gentleman bestows on the self-registering apparatus of No. 2, 

 (deeming it very well worthy of attention and likely to be adopted, if 

 the difficulties before stated in the syphon tube can be obviated), as it 

 works on the same principle as that proposed by myself, induces me to 

 hope that something may be yet done to bring the matter to a success- 

 ful issue. For observatories and fixed stations a syphon tube of glass, 

 or semi-transparent porcelain (as suggested by a friend), will at once 

 remove the objections justly stated by Mr. Taylor, and in my first letter 

 this recommendation was accidentally omitted, for at that time my 

 attention was more particularly turned to the rendering barometers, 

 if possible, indestructable , and more useful to travellers. The drawing 

 annexed, (PI. 4. fig. 1.) will, I really think, serve to show that an iron 

 syphon tube may be so constructed as to be easily filled and carried 

 about without the chance of air getting either into the longer limb, or 

 into any situation from which it will not be immediately expelled, when 

 properly prepared for action — (vide figure and remarks). By tapping 

 the barometer w)|ile horizontal before working it, the ^air bubbles will 

 rise to the surface near the stop cock ; for since the cistern was nearly 

 full of mercury originally, no additional quantity of air can ever enter 

 it, since the stop cock will be made mercury tight : the passage of air 

 through it will be advantageous. There is room also for the mercury 

 to expand in high temperatures, and after a journey, when the instru- 

 ment is laid horizontally, trifling particles of air, which may adhere to 

 the sides of the cistern, can be readily expelled by heating the screw 

 end of the cistern after opening the stopper. It will be manifest, 

 also, on inspection before placing the barometer vertically, whether 

 much air has entered the instrument, for the mercury to allow of such 

 occupation must necessarily have been forced through the stop cock or 

 screw ; but this seems an impossible case, while the air itself can be so 

 much more easily pressed through those places. 



From trials with an iron barometer in my possession, I think the long 

 tube should be of one uniform bore throughout, and not larger at the 

 vacuum end, as before recommended by me, unless made by the most 

 careful and expert workman, since the difficulty of boring the latter, 

 and the trouble of expelling air from it, become very great ; while the 

 scale also requires more than ordinary attention. 



