434 Dr. G. F. Barker on a new 



If the scale on which it is carried out is sufficiently small to 

 permit of the faces being made of cardboard, it would, I think, 

 answer very well. We should then have to select the rive 

 appropriate sheets (each comprising four faces) and mount them 

 on the umbrella stand ; the five sheets would then represent one 

 quarter of the globe. 



The map-sheets may be kept in a very small compass, because 

 the isosceles triangles may be folded down over the equilateral 

 triangles, as shown in fig. 7. 



In the other scheme it requires six or seven sheets to represent 

 nearly one fifth of the globe : but it has the countervailing ad- 

 vantage of permitting a greater choice of the region which is to 

 be in the middle ; for, by having two umbrella stands, we can 

 either place a pentagon or a hexagon in the middle. 



In the instrument as made for the Geographical Society, the 

 same general framework serves for both umbrellas, which may 

 be shifted with great ease. It was this advantage in choice of 

 the central region displayed which induced the Committee to 

 prefer the original 32-faced polyhedron. 



A similar construction may of course be applied to a dodecahe- 

 dron inscribed in a sphere ; and we thereby obtain a 72-faced sur- 

 face, viz. 12 pentagons, and 60 obtuse-angled isosceles triangles. 

 Here, as before, the map-sheets might be printed in sets of six, 

 viz. a pentagon surrounded by five triangles ; three map-sheets 

 will then give one quarter of the globe. In this figure the 

 pentagons are so large compared with the triangles that the 

 approximation to the sphere is not very close. 



Models of the various plans explained were exhibited at Brad- 

 ford ; but, by an oversight, no abstract of this paper appeared 

 in the British- Association Report. 



LII. On a new Vertical-Lantern Galvanometer. By George 

 F. Barker, M.D., Professor of Physics*. 



DESIRING to show to a large audience some delicate ex- 

 periments in magneto-electric induction, in a recent lecture 

 upon the Gramme machine, a new form of demonstration 

 galvauometer was devised for the purpose, which has answered 

 the object so well that it seems desirable to make some per- 

 manent record of its construction. 



Various plans have already been proposed for making visible 

 to an audience the oscillations of a galvanometer-needle; but 

 they all seem to have certain inherent objections which have 

 prevented them from coming into general use. Perhaps the 

 most common of these devices is that first used by Gauss in 



* Communicated by the Author, having been read before the American 

 Philosophical Society, May 7th, 1875. 



