| 释义 |
periphractic, a. Geom.|pɛrɪˈfræktɪk|
[mod. f. Gr. περίϕρακτ-ος fenced round (f. περιϕράσσειν, f. ϕράσσειν to fence) + -ic. (Orig. app. in Ger. by Listing.)]
Said of a region having one or more internal bounding surfaces (or curves, when the region is plane) unconnected with the external boundary (e.g. a globe with an internal cavity, or a circular race-course round an enclosed space), so that a closed surface (or line, when the region is plane) may be drawn within the region, such that it cannot be contracted to a point without passing out of the region. (Cf. cyclic 5.) Hence transf. as periphractic number, the number of independent internal boundaries in such a region.
1881Maxwell Electr. & Magn. I. 17 When a region encloses within itself other regions, it is called a Periphractic region. Ibid. 24 The most familiar example of a periphractic region within which the solenoidal condition is satisfied is the region surrounding a mass attracting or repelling inversely as the square of the distance. Ibid. 23 The whole number of lines to be drawn to remove the periphraxy is equal to the periphractic number or the number of internal surfaces. 1895H. Lamb Hydrodynamics 43 Let us suppose that the region occupied by the irrotationally moving fluid is periphractic, i.e. that it is limited internally by one or more closed surfaces. |