59 



wards subjected to a second calculation by others not connected with 

 them, is such as to prevent the introduction of accidental errors, and 

 to remove altogether the possibility of tampering with the observa- 

 tions. It therefore allows implicit confidence to be placed in the 

 work. 



The measurement of the more recently selected bases has been 

 made by a new apparatus, contrived by the Superintendent for the 

 purpose, (the description of which will be found in his Report for 

 1847, and in the Proceedings of this Society, Vol. IV. p. 368,) in 

 which the arrangements for the support of the bars, compensation 

 for temperature, accuracy of contact and delicacy of alignment, are 

 such as, in the opinion of the Committee, leave nothing to be 

 desired,' while the apparatus is so simple, and so easily manipulated, 

 as to allow of a rapidity of work which, it is believed, has never be- 

 fore been obtained. The measurements, by this instrument, are, of 

 course, verified by the most severe and improved methods, and the 

 agreement of the various measurements, some of which were made 

 under circumstances peculiarly unfavourable, shows a degree of ac- 

 curacy which has certainly never been surpassed. The application 

 of the electric telegraph to the determination of differences of longi- 

 tude, and the invention of a clock by which the time is accurately 

 registered without the intervention of any assistant, at stations at any 

 distance apart allows the correct difference of time between two 

 places to be determined within a very small fraction of a second, and 

 will give the means of getting the differences of longitude between 

 our principal cities with a degree of accuracy which has only been 

 obtained at the principal observatories of Europe, after many years 

 of continued observation and comparisons. 



Improvements upon the astronomical apparatus used, and careful 

 experimental observations upon the relative efficiency of different 

 forms of instruments, and different modes of observing, have led to 

 great accuracy in the observations for the determination of latitudes, 

 so that the places of the smaller stars, as given in the catalogues, 

 are not now sufficiently precise for the work, and the positions of the 

 stars used have to be determined by direct observations. 



The arrangements for having the maps, after reduction, engraved 

 in the office of the survey, under the immediate inspection of an assist- 

 tant, and then, after two separate verifications, submitted to the Super- 

 intendent, before publication, are calculated to maintain the accuracy 

 of the field-work upon the map issued, as far as that is possible. 



The charts issued from the office bear the marks of the great 



