610 ON THE FIGURE, DIMENSIONS, AND MEAN SPECIFIC GRAVITY OF THE EARTH, 
From the pendulum experiments at a great depth and on the surface, the Astro- 
nomer Royal obtained 6‘566. 
I have recently received, through the Astronomer Royal, two copies of the new 
National Standard Yard ; it is obviously necessary that our geodetic measures should 
be given in reference to the standard ; but not knowing from what scale the standard 
has been taken, I am unable to say at jjresent in what way the reduction is to be 
made; that is, whether by reference to the comparison of the old standards which 
have been ali eady made, or by the mechanical process of a direct comparison of the 
Ordnance Standard with the new National Standard. 
H. James, Lieut. -Colonel R.E. 
Tlie Principal 'Friangulation of the Oi’dnance Survey of Great Britain and Ireland, 
extending from the Scilly Isles, in latitude 49° 53', to the Shetland Isles, in latitude 
60° 50', and embracing at its widest extent about 12° of longitude, consists of about 
250 stations. 
The observations for the connexion of the trigonometrical stations have been made 
with four large theodolites, two of 3 feet, one of 2 feet, and the other of 18 inches in 
diameter. The first two instruments (one of which is the property of the Royal 
Society), and the 18-inch theodolite, were constructed by Ramsden at the com- 
mencement of the trigonometrical opeiations in England in 1798: the 2-feet theo- 
dolite was constructed by Messrs. Troughton and Simms at the commencement of 
the Irish Survey in 1824. 
The latitudes of thirty-two of the stations of the principal triangulation have been 
determined by observations made with Ramsden’s zenith sector, and since the 
destruction of that instrunsent in the great fire at the Tower of London, with Airy’s 
zenith sector. All the observations made wdth these instruments have been published 
in detail*. 
The mode of observing with the theodolite may be shortly described as follow's: — 
Tlie instrument being first placed very carefully over the precise centre of the station, 
an object having a fine vertical line of light, with a breadth of about 10", is set up in a 
convenient position wdthin a mile or two of the station ; this object, called the 
“ referring-object,” serves as a point of reference from which all angles are measured. 
The lower limb of tlie instrumetU being clamped, the observer intersects the refer- 
ring object and then each of the pi'incipal points in succession, concluding with a 
second observation of the I’eferring-object, which should be identical, witliin the 
limits of errors of observation, wdth the first reading of that object: the instrument 
is then unclamped and the bearings read again on different parts of the divided 
circle. 'I’he method by which these observations are reduced to the most probable 
* ‘Astronomical Observations made with Ramsden’s Zenith Sector,’ 1842. ‘Astronomical Observations 
made with Air.v’s Zenith Sector,’ 1852. 
