Xll 



sixteen times. In the summer of 1862 the longitude of Valentia was 

 again determined, this time by the use of the electric telegraph. The 

 electric telegraph was also used in 1853 to determine the differences 

 between the longitudes of Greenwich, Cambridge, and Edinburgh, 

 and, when the submarine telegraph was laid to Ostend, the difference 

 of longitude between Greenwich and Brussels. These differences of 

 longitude have been combined with other Continental determinations, 

 and, by the use of the whole series, the longitudes of places in the 

 extreme east of Europe may be compared with places in America. 



In the year 1844 Mr. Airy also assisted in tracing the Canadian 

 boundary under the Treaty of Washington. The corps of Royal 

 Engineers who were to mark the boundary were placed at the Obser- 

 vatory for instruction and practice in the use of instruments under 

 his eye. The most difficult part of the boundary was a straight line 

 of nearly seventy miles in length, passing through a country of 

 impervious forests, steep ravines, and dismal swamps. He 

 arranged a plan of operations founded on a determination of the 

 absolute latitudes and the differences of longitudes of the two 

 extremities. The azimuths of the line for the two ends were then 

 computed, and marks laid off for starting from each end. One party 

 of engineers, after cutting more than forty-two miles through the 

 woods were surprised, on the brow of a hill, at seeing a gap in the 

 woods, on the next line of hill ; this turned out to be the line of the 

 opposite party. On continuing the lines until they passed abreast of 

 each other, their distance was found to be 341 feet. This corresponds 

 to an error of only a quarter of a second of time in the difference of 

 longitudes, and is about one-third of the error which would have 

 been committed if the spheroidal form of the earth had been 

 neglected. This is a striking testimony both to the accuracy of Mr. 

 Airy's method and to the skill of the engineers. 



At a special meeting of the Board of Visitors in 1843, Mr. Airy 

 proposed to construct the first of the important instruments he has 

 added to the Observatory. He points out that the Royal Observatory 

 was instituted chiefly to observe the Moon, and that this object had 

 been so continuously kept in view ever since its foundation, that the 

 existing theories and tables of the Moon aie founded entirely on 

 Greenwich observations. The unavoidable interruptions to the 

 regularity of the series were, however, so numerous, that the number 

 of complete observations then made was under a hundred a year. He 

 proposed to supply this deficiency by erecting an altitude and 

 azimuth instrument, by which the Moon could be observed in any 

 part of the sky. He assures the Visitors that its cost ought not to be 

 an objection when it is remembered that each complete lunar obser- 

 vation was then worth ten pounds. In the report of 1847 we read 

 that the new instrument had been completed, and was in working 



