C Barus — Counter-twisted Curl Aneroid. 115 



Art. XV. — The Counter-twisted Curl Aneroid /* by Carl 

 Barus, Hazard Professor of Physics at Brown University. 



Introductory. 



1. — It seems plausible to argue that much goes on in the 

 atmosphere immediately related to pressure which the ordinary 

 pneumatic barometer merely integrates, and of which it can give 

 no detailed account. I refer both to the changes referable to 

 the gusty character of the wind f and to pressure variations of a 

 more subtle nature,^ the origin of which may be considered in 

 relation to the earth's magnetic and electrical field. 



2. — The problems, therefore, are beyond the immediate 

 scope of instruments of large mass like the ordinary mercury 

 or water barometers. The conditions to be fulfilled are (1) 

 great sensitiveness and (2) instantaneous indications ; (3) regis- 

 try subject to corrections of a purely scientific kind. It is 

 when these three conditions are simultaneously demanded that 

 the problem becomes formidably difficult. No matter what 

 form of mechanism is selected, one is brought face to face at 

 once with viscosity and with the thermal variations of both vis- 

 cosity and of elasticity. 



I desire in this paper to find out how far one can go with 

 suitable modifications of the Bourdon tube. In certain meas- 

 urements! of high pressure made with such a tube coiled heli- 

 cally I received much encouragement, inasmuch as the instru- 

 ment could be read off closely enough, without the aid of 

 subsidiary mechanism. The difficulties which I then encoun- 

 tered were purely technical. In flattening and coiling the 

 necessarily heavy tube, I had to remove the temper at a sacrifice 

 of strength and resilience, and the gauge actually burst at 1000 

 atmospheres. I abandoned it simply because of the difficulty 

 in fashioning this cumbersome apparatus in the laboratory. 



In relation to low pressures, however, all of these difficulties 

 fall away at once, and it becomes merely a question of patience 

 to find the limit of constancy and precision to which a gauge 

 of this kind can be pushed. 



The conditions of sensitiveness were discussed not long ago 



* The present research was encouraged by a fund kindly placed at my disposal 

 by the Secretary of the Smithsonian Institution. 



f Cf. S. P. Langley: "The Internal Work of the Wind," Smithsonian contribu- 

 tions, No. 884, 1893, Washington, D. C. 



% Considerations of this kind originated, I believe, with the late Prof. Balfour 

 Stewart. Recently the subject has been attacked more seriously, notably by 

 Prof. F. H. Bigelow. 



§ Barus: Bulletin U. S. Geological Survey, No. 96, p. 29, 1892. Cf. Proc. 

 American Acad., xxv, p. 106, 1890. 



