837 
Measurement of three Degrees of the Meridian . 
construction, or some accidental derangement, there is an 
error of some seconds in the observations of the fixed stars. 
How is this to be discovered ? This is not to be done by com- 
paring the value of a degree on the meridian, as deduced from 
these observations, with the results of other measurements in 
distant parts of the globe. For if we find that these degrees 
so taken do not agree in giving the same ellipsoid, we are not 
to attribute all the differences to irregularities of the earth, 
without supposing any error on the part of the observer, of 
his instrument, or of other means employed in his survey. 
But this, in fact, is what has generally been done. It must, 
however, be acknowledged, that the majority of observers 
have not been in fault, as they could do nothing better ; but 
too much reliance has been placed on the goodness of their 
instruments, their means, and other circumstances. It is true 
that irregularities of the earth and local attractions may occa- 
sion considerable discrepancies which are even inevitable ; but 
before we decide that these are the real source of disagree- 
ment, we ought carefully to ascertain that there are no others. 
But to return to our subject, of the English measurement. 
If the uncertainty which yet subsists, with respect to the exact 
figure of the earth and its dimensions, occasions some small 
errors in the calculation of the series of triangles, the sum of 
these errors will be found in the estimate of the entire arc, 
and will increase in proportion to the extent of the arc mea- 
sured. Now, in the English measurement, we find exactly 
the reverse of this. For the difference between the results of 
calculation and observation is only i", 38 on the whole arc; 
but is even as high as 4/', 77 on one of the smaller arcs. So 
that, whatever error we may suppose to have been introduced 
