342 
On the Measurement of the 
[Nov. 
the danger of shaking the register marks by driving the stretching posts in the 
ground: these were in some measure compensated by the rapidity of passing over 
the line in spaces of 100 feet at a time. The line at Misterton Carr of 26,342 feet in 
length, measured by the chain, is not supposed to err more than two inches from 
the truth. 
The French academicians made use of double rods of platina and brass, 12 feet 
long, so fitted tegether that from the unequal expansion of the two metals, a register 
pointed out at one end the mean temperature of the whole apparatus, and the 
consequent correction for temperature was easily applied. 
But Colonel Colby has lately introduced, in the Irish survey, a compound bar 
which nullifies all the corrections for heat, and provides in the most perftct way 
for the several adjustments of horizontality, colliination, and juxtaposition ; at the 
expence, it must be confessed of some little increase of trouble in the field, though 
on the other hand it saves all calculations of reduction, a principle to which 
English artists seem particularly attached. This is the form of measuring rod now 
adopted in the Indian Trigonometrical survey, in fact the apparatus brought out 
is the exact duplicate of that invented by Colonel Colby. 
Two bars, one of steel, the other of brass, are placed side by side in a long wooden 
box, and united firmly together by their middle ; they are also connected at their 
extremities by levers or cross bars, projecting on the side of the steel bar to about 
twice the distance between the bars : the levers carry very fine dots imprinted on 
silver discs, so adjusted, that the difference of expansion of the brass and steel 
keeps the dots always at the same distance apart. This adjustment is experimentally 
made by submitting the bars to alternate heat and cold in a proper apparatus : but 
it can easily be calculated from the known expansions of steel and brass, which 
are nearly as 2 to 3. The bars are enclosed in a double case of flannel and deal, 
resting upon rollers to allow of free motion : they are 9£ feet long ; and there 
are six pair, so that when placed on tripods, six inches in advance of one another, 
they occupy a length of 60 feet. The avoiding of contact between the ends ot the 
bars is essential to their steadiness : the six intermediate inches are read off by 
means of double microscopes, connected firmly together, which are also provided 
with compound bars for compensation of temperature. In the microscope at t 
two extremities of the series of bars, a third microscope is placed between the 
other two, having a longer focus for the purpose of reading off a minute dot on 
a platina disc fixed on a steady brass tripod, which is called the register, maintain- 
ing its post while the bars are moved on. We need not here advert to them" 
ingenious contrivances for levelling, moving, and protecting the apparatus, &■ 
hope on some future occasion to be permitted to give our readers a drawing 0 
whole apparatus. ^ 
The base measured by Colonel Colby, with these bars, near Loch F° J l e > ren£ 
land, was submitted to the most rigorous examination by comparing the ^ ^ 
paits together, and the extent of variation was found to be only 3 * nC ^\ c j orS) 
many miles. The Indian bars were also tried by order of the Court of ir ^ ^ 
in Lords’ cricket ground, and the discrepancy between two measuremen ^ 
line of 567 feet was only 3-40ths of an inch. Any one who lias witnessed ^ to f 
operandi with them on the Barrackpfir road, will easily believe the atta ^ gS 0 f the 
an equal or even greater precision here, from the firmness and evenn ^ 
ground, the steadiness of the climate, and the host of experienced sui^ejor 
in carrying it forward. ^ w j,icb |S 
II- Next to the base in importance comes the system of triaNGULAT itself? 
however so far independent, that it contains a principle of verification ** - s 0 { 
well known to every geometrician. Where accuracy is so indisp ensa 
