248 



gray: linear expansivity 



Areas Within Which Tropical Cyclones Originate 



Conditions favoring the formation of the West Indian and 

 other cyclonic systems in the tropics are produced by changes 

 in the positions and intensities of the so called permanent areas 

 of high and low atmospheric pressure incident to seasonal changes 

 and to variations in the intensity of isolation. Hence there 

 are three and only three regions, as listed above, all more or less 

 similar, in which destructive tropical storms originate. Once 

 formed these systems are carried along in the general drift of 

 the atmosphere — from east to west below about latitude 30°N., 

 and from west to east in higher latitudes. 



PHYSICS. — New methods for displacement measurements and 

 temperature uniformity applied to the determination of linear 

 expansivity. Arthur W. Gray. Bureau of Standards. 

 Communicated by L. A. Fischer. 



This communication describes certain improvements in two 

 of the fundamental operations involved in the determination of 

 linear expansivity, viz., (1) The measurement of displacements 

 that occur within places difficult of access, and (2) The produc- 

 tion of uniform temperature thruout an extended region. The 

 paper concludes with pointing out how the methods set forth 

 might find a special application in the problem of gas thermome- 

 try. Some features were outlined at the Washington meeting 

 of the American Physical Society in December, 191 1. 1 



The measurement of inaccessible displacements. Essentially the 

 method consists in determining the displacement of an inaccessi- 

 ble point by observing a stretched wire so arranged that its 

 motion follows that of the point in a known way. In order to 

 eliminate from the measurements any elongation of the wire, 



J A. W. Gray, Phys. Rev. 34: 139. 1912. 



