132 Royal Society. 



is but little influenced by the extent of surface. With fibrous sub- 

 stances, such as cloth, the friction diminishes by an increase of 

 pressure, but is greatly increased by the surfaces remaining for a 

 certain time in contact : it is greater, cceteris paribus, with fine 

 than with coarse cloths : the resistance is also much increased by an 

 increase of surface. With regard to the friction of different woods 

 against each other, great diversity and irregularity prevail in the re- 

 sults obtained : in general, the soft woods give more resistance than 

 the hard woods : thus, yellow deal affords the greatest, and red teak 

 the least friction. The friction of different metals also varies princi- 

 pally according to their respective hardness ; the soft metals produ- 

 cing greater friction, under similar circumstances, than those which 

 are hard. Within the limits of abrasion, however, the amount of 

 friction is nearly the same in all the metals, and may in general be 

 estimated at one-sixth of the pressure. The power which unguents 

 have in diminishing friction, varies according to the kind and the 

 fluidity of the particular unguent employed, and to the pressure 

 applied. 



The paper is accompanied with drawings of the apparatus used ; 

 and the details of the experiments are given at length in a tabular 

 form. 



June 19. — " On maenetic influence in the solar rays," by S. H. 

 Christie, Esq. M.A., FTR.S., &c. 



From the experiments described by the author in a former paper, 

 it appeared that a magnetized needle vibrated under exposure to the 

 sun's rays, came to rest sooner than when screened from their influ- 

 ence : that a similar efl^ect was produced on a needle of glass or of 

 copper ; but that the effect on the magnetized needle greatly ex- 

 ceeded that upon either of the others. In the prosecution of this in- 

 quiry, the author has endeavoured to vary the experiments, so as to 

 obviate several causes of inaccuracy, which might tend to invalidate 

 the general conclusions he had before drawn. His first object was to 

 compare the effects of the solar rays on an unmagnetized steel needle 

 with one that was magnetized under the same circumstances ; and 

 the result was, that the latter was influenced in a more considerable 

 degree than the former : and a similar difference was observed when 

 the vibrations of a magnetized needle were compared with those of a 

 needle made of glass or of copper. He ascertained that the diminu- 

 tion of the terminal arc of vibration on exposure to the sun, was not 

 occasioned merely by the heat imparted to the needles or surround- 

 ing medium, although their cause appeared, in some instances, to 

 measure the intensity of the action which produced the diminution. 

 In order to determine the comparative influence of the separate rays, 

 he allowed them to fall on the needles after transmission, through 

 differently coloured fluids and glasses ; but owing to want of oppor- 

 tunity, he was obliged to abandon the inquiry before arriving at any 

 determinate results ; though, as far as they went, they appeared to 

 confirm the conclusion, that the effects were dependent on the degree 

 of light, and not on that of the heat. The red rays, however, ap- 

 peared to have a greater effect in diminishing the terminal arc than 



the 



