162 Proceedings of the British Association. 



escapement, made a difference of fifteen minutes. By Mr. Airy's 

 plan, there was always (if the term may be used) an extra reser- 

 voir of force ; keeping the train of wheels always up to their 

 work, and capable of overcoming the resistance occasioned by 

 the freezing of the oil. Mr. Dent then explained the principle 

 of his patent Compensation-balance. Mr. Frodsham made some 

 remarks on the compensation balance of chronometers, and ex- 

 plained a new balance of his invention. Sir Thomas Brisbane 

 said, that praise was due to Mr. Dent as the first maker who had 

 exerted himself to determine the difference of meridians by chro- 

 nometers. He had shown that by chronometers the difference 

 of longitude could be had with as much certainty as by any 

 other method in use, and at an expense bearing no proportion to 

 that of rockets, or any other means hitherto adopted. Dr. Rob- 

 inson, of Armagh, was at present engaged in a series of rocket 

 observations in Ireland. It had been the intention of Dr. R. to 

 connect the Irish with the Scotch observatories, and for that pur- 

 pose a large deposit of rockets had been obtained from govern- 

 ment, and stood in Dumbarton Castle ; but unfortunately the un- 

 favorable weather in spring had prevented the execution of the 

 design, and he had received a letter within a few days from Dr. 

 Robinson, stating that the strong twilights of the present season 

 would make it requisite to postpone the work until autumn: 

 these facts would at once show the superior economy and saving 

 of time to be attained by adopting Mr. Dent's suggestion of chro- 

 nometrical observations. Mr. Holden inquired, why the method 

 of moon-culminating stars, which was so simple and easy of ap- 

 plication, was not preferred to any other in determining longi- 

 tudes. Sir Thos. Brisbane replied, that not to say nothing of the 

 heavier amount of labor required in such observations, he need 

 only, in order to show the superiority of Mr. Dent's method, 

 state the fact, that in a late attempt to connect the Royal Obser- 

 vatories of London and Paris, backed by all the instrumental 

 accuracy and unrivalled skill of the observers at these two dis- 

 tinguished observatories, three hundred observations on moon 

 culminating stars had given a mean deviating no less than thirty 

 seconds from the truth. The President observed, that although 

 the method of moon-culminating stars had in theory promised 

 considerable accuracy in the determinations of longitudes, yet 

 from some unexplained difficulties, it had in practice fallen far 

 below the estimate that had been formed of it. 



