350 
ISIR. EVANS ON THE DEVIATION OF THE COHPASS 
observations, the results of which, so far as they relate to the early changes in the devia- 
tions of the compass, and the external magnetic lines of the ship, will be found in 
Table V., and an accompanying diagram in Plate XY. 
In the progress of building (18th August, 1857), I ascertained by astronomical bear- 
ings, with the known variation of the compass at the time, that the magnetic direction 
of her keel and head was S. 29°50'E. A short time previous to the launching 
(November 1857), I also obtained the few results for the external magnetic lines, which 
will be found in the diagram. 
After launching, the Great Eastern was secured near Deptford dockyard, with her 
head S. 46° E. magnetic; ditfering only 16° from the original dii'ection while building; 
considerable works, chiefly internally, were in progress till the time of quitting this 
anchorage. In August 1859, a few days prior to lea’sing the river, an extended series 
of observations for the development of the external magnetic lines (see diagram, 
Plate XV.) was obtained; which show that some alteration had taken place since the 
first approximate determination. 
On September 7th, 1859, the Great Eastern quitted Deptford on her preliminary 
trip to Portland, arriving at the latter port on the 10th. On the passage between these 
places, which included temporary anchorages at Purfleet and the Nore, eveiy opportunity 
was embraced of determining the deriations of the compass at various selected stations 
in the ship which are marked in the diagram. The deriations whilst in the River 
Thames were determined by noting the difference between the bearing of tlie ship’s 
head, as astronomically determined (by the use of a repeating card, and prepai'ed table 
of the sun’s bearings), and as denoted by the compass : a curve of deviations embracing 
each point of the compass was then graphically drawn through the general results. 
At Portland, lying Avithin the breakwater, the correct magnetic bearing of St. Alban’s 
Head, a distant point of land, afforded ready means of determinmg the deAiations as the 
vessel SAvung round at her anchor ; and subsequently at Holyhead, AA'here I proceeded 
again in conjuction with Mr. Rundell, similar means Avere adopted as at Portland by 
the bearing of a distant Welch mountain. As it AA’as not possible to “sAving” the 
Great Eastern, from her ponderous bulk, in the mode usually adopted under these 
circumstances Avith ordinary sized vessels, Ave were dependent on the changes of ship’s 
head arising from Avinds and tides : this necessarily occupied much time, and was the 
occasion of gaps being left in the cuiwes of deA’iations obtained both at Portland and 
Holyhead, Avhich required to be arbitrarily filled up. As these ciuTes presented no 
anomalous differences from those of ordinary sized ships, but were marked by perfect 
regularity and consistency among the observations, these gaps Avere confidently com- 
pleted. 
From a consideration of the magnetic character of the Great Eastern, it is apparent 
that the Admiralty Standard compass, as also the ship’s Standard compass, were placed in 
the best position Avith respect to the hull and machinery, both for experimental research 
and the navigation of the ship ; indeed, from the vast amount of machinery and neces- 
