1868.] 370 IBriggs. 



which is, probably, at least as great as that of liglit, at the sur- 

 face of the magnetized body. 



17. Terrestrial and cosmieal magnetism can be satisfactorily 

 accounted for by the hypothesis that all the magnetic eddies 

 are produced by the action of gravity in the restoration of dis- 

 turbed equilibrium. No known force, other than gravity, has 

 :i known velocity sufficient to enable it to act pronii)tly, as an 

 equililjrating force, in all the known cases of magnetic disturb- 

 ance. 



Mr, Briggs addressed the Society, and stated that — 



He had recently been considering the subject of construction 

 of domes, having been led to do so b}'' the proposal to inclose 

 and cover one of the Penn Squares (228 feet square), by some 

 temporary and light, but si^acious building, for an exhibition of 

 manufacturing industry by the Franklin Institute. He had 

 conceived the plan of a structure, the main portion of which 

 should be a hemispherical dome of 208 feet diameter, and this 

 dome he had designed to be a framework in carpentry, with- 

 out tie-rods or internal support, but rel^ang for its strength 

 and stability upon tie-bauds, which, should be adequate to carry 

 the horizontal radial components which existed as tension or 

 compression at the top and bottom ends of the rafters. The 

 contour of the dome proposed, was rather a funicular polygon 

 than a semicircle, but the study, when reduced to the general 

 case, involved all forms of what might be termed a homoge- 

 neous dome. 



The particular instance and application, Mr. Briggs said he 

 had already endeavored to elucidate in as popular a way as he 

 could, at a meeting of the Franklin Institute on Wednesday 

 last, and a report of the remarks would appear in the Journal 

 of the Institute at an early day, but what he wished to call the 

 notice of the members of the Philosophical Society to, was that 

 the general investigation of the equilibrium and strength of a 

 homogeneous dome of an egg-shell had never been considered. 

 The dome of equilibrium of our treatises on applied mechanics, 

 was a transformed catenary where the load on the chain varies 

 as (p/.^ in place of as a constant. This curve has the same 

 characteristic constant horizontal component as the ordinary 

 catenary ; and if a dome be constructed of this form, each part 

 is in equilibrium, and the entire strain is to be carried by the 



