Improvement of the Barometer. 99 



point, which renders the column less liable to admit air, either from 

 a concussion or inverting its position. 



This improvement, alone, I deemed of sufficient importance to jus- 

 tify the construction of a new barometer, and was actually prosecu- 

 ting it, when an idea of the globe suggested itself to me. 



In all the portable barometers that I have seen, the end of the 

 tube is cut or broken off in a careless manner, which as often leaves 

 it concave as convex, and it must be apparent to every one who will 

 examine the subject, that bubbles of air striking the concave end of a 

 straight tube, are more likely to enter the column than to roll off. 



k is the globe, f of an inch in diameter, fixed firmly on the tube 

 at /, and has a very small aperture at m, the only place where the 

 mercury inside can communicate with that in the cistern. The globe 

 is of cast-steel, with which mercury is known to come in perfect con- 

 tact ; consequently, the atmospheric pressure cannot force the air 

 through this aperture nor through the bottom of the tube. By examin- 

 ing this arrangement, you will perceive the impracticability of even 

 forcing the atmosphere through the globe, much less the possibility of 

 its being driven there by inverting and re-inverting the instrument, 

 or by any jar or concussion which it may receive while in use or be- 

 ing transported. 



Even admit air to be placed in the globe, it is apparent that it 

 would find its way out by the aperture, one hundred times oftener 

 than it could possibly enter the tube. 



n is the leather bag through which the atmosphere coinmunicates 

 Us influence to the whole interior of the cistern and column. Leath- 

 er is the most in use, although there are other methods to admit the 

 atmospheric pressure, in forms of the instrument, which are perhaps 

 not so portable ; although for general use, a short lube, with a stop- 

 cock or plug inserted in the top of the cistern, is probably the best. 



represents a screw-joint of the cistern, where it is separated 

 while attaching the globe to the tube ; for the tube being connected 

 with the cistern at p, would render it impracticable to fasten the 

 globe without this separation. I mention these minutiae, because 

 those who undertake to make a barometer on this plan, may other- 

 wise be subject to the same perplexities which I experienced in the 

 construction of mine. 



1 did not succeed in obtaining a globe, until the third person made 

 the attempt, and produced it from a solid piece of steel. If made in 

 two parts, it would necessarily be joined with solder, on which the 

 mercury would act too powerfully. 



