what has since grown into first-rate importance as the Topographical 

 Department. The task at that time was beset by difficulties, which the 

 progress of physical and mechanical science has since removed : the pre- 

 paration of the base-apparatus, the construction of astronomical and other 

 surveying-instruments, the contriving of signals by lamp and heliostat, and 

 the training of sappers for their special duties had to be undertaken under 

 the disadvantage of newness. But at that time the Duke of Wellington 

 was Master- General of the Ordnance ; and supported by him, Colonel Colby 

 carried out his plans in full efficiency. 



In 1825 the first detachments were removed to Ireland, and the first 

 trigonometrical station was taken up on Divis Mountain, near Belfast. 

 There the first signals and observations with lamp and heliostat were 

 attempted, and, to the satisfaction of the originators, proved completely 

 successful. This was Lieut. Portlock's start on the trigonometrical branch 

 of the survey, of which he shortly became the senior, and eventually sole 

 officer. 



In addition to scientific skill and accuracy, great personal endurance 

 was required in carrying on the observations. In 1826 the camp on Slieve 

 Donard, 2800 feet above the sea, was more than once blown down by the 

 violence of the wind. Colonel Colby was seriously injured by a fall while 

 climbing from the observatory to his tent ; and communication with the 

 country below involved both difficulty and danger. Yet " Portlock," we 

 are told, " held out to the last. For some weeks he was the only officer 

 remaining ; but he struggled on, and brought the operations to a successful 

 close." 



In the following year, while Colonel Colby was measuring the base on 

 the shore of Lough Foyle, Lieut. Portlock, with Lieut. Larcom, carried 

 out the observations at seven hill-stations, regardless of season and weather. 

 In 1828, and for some years afterwards, he performed the work single- 

 handed, observing with the great theodolite from mountain after mountain 

 till the principal network of triangulation was complete, and the Irish 

 system was, by means of the lamp and heliostat signals, united to that of 

 Britain. In addition, care had to be taken for the direction of the secondary 

 triangulation for the details of the survey, and for the rectification of 

 errors and the discrepancies that were sure to occur at the junction of the 

 separate districts. For this the whole had to be combined under one 

 general system ; and this additional labour Lieut. Portlock undertook 

 while still on the mountains. He carried it on afterwards at his office in 

 Dublin ; and so well did he direct these secondary operations, that, after 

 the parties became used to the work, the surveying went on at the rate of 

 three million acres a year. 



The horizontal survey involved the necessity of an elaborate vertical 

 survey and calculations for altitude. The altitudes were deduced at first 

 from the sea, by actual levelling from it to bases of altitude, and from them 

 transferred, by angles of elevation and depression, to the summit of every 



