94 Mr. J. T. Bottomley on Expansion with Rise of 



light upon this also. In my own negative the point of 

 crossing seems to be extra bright. 



Meanwhile 1 must apologize to the Society for bringing 

 forward these notes in such an immature and hastily con- 

 structed condition. My excuse must be that the photographs 

 of electric sparks were only taken the day before yesterday, 

 and today's meeting is the last of the session. 



XIII. Expansion icith Rise of Temperature of Wires under 

 Pulling Stress. By J. T. Bottomley, M.A., F.R.S., 



F.C.S.* 



[Plate IX.] 



IT is probably well known to the members of the Physical 

 Society that, at the instance of the British Association 

 and with the assistance of a money grant from that body, 

 very interesting secular experiments on the elasticity and 

 ductility of wires were commenced some years ago in 

 Glasgow. In the tower of the Glasgow University buildings 

 certain wires are hung in pairs for comparison. One of each 

 pair carries a heavy load about half the breaking weight of 

 the wire ; the other carries about one tenth of the breaking 

 weight. Certain marks are put on the wires; and the object 

 of the experiment is to find whether the heavily loaded wire 

 seems, on comparison with the lightly loaded wire, to go on 

 running down incessantly, or whether it comes asymptotically 

 to a fixed length for a given temperature, ceasing to ex- 

 perience further permanent elongation. 



The observations of the last few years show that the 

 elongation due to further pulling out has, to say the least, 

 become exceedingly small, so small that it is extremely 

 difficult to observe it ; and at the Aberdeen meeting of the 

 British Association I pointed out that a great difficulty is 

 introduced into the making of deductions from these observa- 



one. The experiments point to the conclusion that diffused light acting 

 upon a plate can reverse previously impressed images of electric sparks, 

 but is powerless to affect any such impressions which may be made after- 

 wards. Similar results are obtained whether the source of the diffused 

 light is a gas-flame, a lamp, or a series of sparks. I do not at present 

 offer any theoretical explanation of these f acts, but they are in themselves 

 sufficient from a meteorological point of view. "Dark " flashes of light- 

 ning have no existence in nature, but are caused by the exposure of the 

 plate to an illuminated sky after the passage of the flash. This illumi- 

 nation may be due to subsequent flashes, the more recent of which will 

 give normal images possibly crossing the reversed ones. 



* Communicated by the Physical Society : read June 22, 1889. 



