FROM THE ORDNANCE TRIGONOMETRICAL SURVEY. 
611 
results, is an approximate solution of the equations resulting from the method of 
least squares. 
The direction of the meridian has been deteriiiined by observations of tlie elonga- 
tions of a, X Urs. min. and 51 Cepljei ; at six of the stations at which these observa- 
tions have been made, the probable error of the result is under 0"'40, at twelve under 
0"'50, at thirty-four under 0"'70, and at fifty-one under 1" 00. 
Measured Base Lines. 
The account of the measurement with Ramsden’s steel chain of the base lines on 
Hounslow Heath in 1791? Salisbury Plain in 1794, on Misterton Carr in 1801, and 
on Rhuddlan Marsh in 180G, will be found in the ‘Account of the Trigonometrical 
Survey.’ These base lines are all expressed in terms of Ramsden’s brass scale at the 
temperature of 62° Fahrenheit. The chains were compared with Ramsden’s pris- 
matic bar (20 feet in length), which was laid off from the brass scale at the tempe- 
rature of 54° Fahrenheit. By a series of comparisons of the Ordnance 10-foot 
standard iron bar (designated O,) with Ramsden’s 20-foot standard, made at South- 
ampton, it was found that 
Ramsden’s bar=20-0007656 feet of Oj, 
so that any measurement expressed in terms of feet of Ramsden’s bar at 62° must be 
multiplied by 1*0000383 to give feet of Oj, at the same temperature. Also to reduce a 
measurement expressed in terms of Ramsden’s brass scale to the same in terms of 
Ramsden’s bar, it must be increased by a quantity corresponding to the diflference of 
the expansions of brass and iron for 8°; and taking these quantities as used in the 
reductiun of the bases, it will be found that the multiplier is 1*0000328, and hence 
to reduce the old bases to feet of O,, they must be multiplied by 1*0000/11. 
In 1816 a liase line of live miles in length was measured by Major-General Colby 
on Belhelvie Sands, Aberdeenshire : the measurement was elfected with Ramsden’s 
steel chains, and in precisely the same manner as the previous bases. The chains 
were compared with Ramsden’s bar by Mr. Berge both before and after the measure- 
ment. 
In 1826-27 the Lough Foyle Base was measured in the north of Ireland with Major- 
General Colby’s compensation bars. Of this measurement a description in detail 
has been published*. 
In 1849 the old base line on Salisbury Plain was remeasured. This measurement 
exceeded the old measure when reduced to the same standard by a foot. The guns 
marking the extremities of the old line were found imbedded very fii’inly in the 
earth, and in all probability in exactly their original positions. 
By a series of comparisons instituted in 1834'|' between the Royal Astronomical 
* ‘ Account of the Measurement of the Lough Foyle Base,’ by Captain Yolland, R.E., 1847. 
t The observations which were made by the late Lieut. Murphy, Royal Engineers, are given in the 
‘ Account of the Measurement of the Lough Foyle Base.’ 
