EXPERIMENTS UPON WROUGHT-IRON GIRDERS. 45 



work, supplied by the Kew Committee, completes the apparatus required 

 for establishing the self-recording system, with the exception of the merely 

 photographic part. It is hoped that this will be completed, under the 

 direction of Mr. Stewart, and the observations of atmospheric electricity com- 

 menced, in little more than a month from the present time. In the mean 

 time preparations for observing the solar eclipse, and the construction of 

 magnetic instruments for the Dutch Government, necessarily occupy the staff 

 of the Observatory, to the exclusion of other undertakings. It is intended 

 that the remaining one of the ordinary house electrometers, with a water- 

 dropping collector, and the portable electrometer referred to above, will be 

 used during the summer months for observation of atmospheric electricity in 

 the Island of Arran. Your Committee were desirous of supplying portable 

 apparatus to Prof. Everett, of Windsor, Nova Scotia, and to Mr. Sandiman, 

 of the Colonial Observatory of Demerara, for the observation of atmospheric 

 electricity in those localities ; but it is not known whether the money which 

 has been granted will suffice, after the expenses yet to be incurred in esta- 

 blishing the apparatus at Kew shall have been defrayed. In conclusion, it is 

 recommended to you for your consideration by your Committee, whether 

 you will not immediately take steps to secure careful and extensive obser- 

 vations in this most important and hitherto imperfectly investigated branch 

 of meteorological science. For this purpose it is suggested, — 1. that, if 

 possible, funds should be provided to supply competent observers in different 

 parts of the world with the apparatus necessary for making precise and com- 

 parable observations in absolute measure ; and 2. that before the con- 

 clusion of the present summer a commencement of electrical observation 

 from balloons should be made. 



Experiments to determine the Effect of Vibratory Action and long- 

 continued Changes of Load upon Wrought-iron Girders. By 

 William Fairbairn, Esq., LL.D., F.R.S. 



Amongst engineers opinions are still much divided upon the question, whe- 

 ther the continuous changes of load which many wrought-iron constructions 

 undergo, has any permanent effect upon their ultimate powers of resistance ; 

 that is, whether a beam or other construction subjected to a perpetual change 

 of load, would suffer such an alteration in the structure of the iron or the 

 tenacity of the joints, that it would in time break with a much less force than 

 its original breaking weight. But few facts are known, and few experiments 

 have been made bearing on the solution of this question. We know that in 

 some cases wrought iron subjected to continuous vibration assumes a crystal- 

 line structure, and is then deteriorated in its cohesive powers ; but we are yet 

 very ignorant of the causes of this change, and of the precise conditions 

 under which it occurs. 



A few experiments were made by the Commission appointed to inquire 

 into the application of iron to railway structures, to ascertain the effect of 

 changes of load upon homogeneous bars of wrought and cast iron. They 

 found with cast iron that no bar would stand 4000 impacts, bending them 

 through one-half of their ultimate deflection, but that sound bars would 



