ON SURVEY AND VALUATION OF IRELAND.
67 surveys will afford an excellent map for a mining engineer to record bis observations on; JKW and it might be possible for many of the surveyors employed, with some instruction W«Ww» Baft.
in ^oology, to become extremely useful assistants to the principal person employed ^
^ in drawing up such a map.
(May 13.)
Has it ever occurred that the neighbourhood of the Killery slate quarries to the sea would make them particularly valuable, especially with such a fine depth of water?
—It has.
Do you consider that they are rendered peculiarly valuable for that reason?—
' From their quality and contiguity to such a fine harbour as the Killery.
Is the slate large?—When
I examined it, as far as I could judge, it appeared;to be so ; I could not well judge of the size.
What is the colour of the Killery slate?—It
is a bluish green.
It overhangs the Killery?—It
does; and the whole mountain is slate.
y
APPENDIX.
AN ACCOUNT of a Trigonometrical Survey of Mayo, one of the Maritime Counties of Ireland.
By jTz'/zVaw _Ba/J, Civil Engineer, Member of the Geological Society, London, and M.R.I.A.
.
Read April 30th, 1821.
.
IN the year 1S09 I was employed by the Grand Jury of Mayo to make a map and sur-vey of iheir county, which was to be drawn and laid down from a scale of three inches to the Irish mile.
The whole of the summer of 1809 was occupied in constructing a geome-trioal map of the environs of Castlebar, and in obtaining a knowledge, of the face of the country, with a view to the planning of the triangles, and the measurement of a base as the foundation of the general geometrical survey and map.
After a diligent search, no plain could be found that would admit of a base of more than two miles in length, except by going through bog.
One of these was adopted, and extended to thirty-one thousand nine hundred feet, which was measured three times carefully over with a h'fty feet chain, using iron pins.
I have drawn out a separate diagram of the triangles calculated from this base, as a comparison with the results obtained from more exact data.
I ordered Mr.
Edward Troughton, of London, to make a chain of one hundred feet in length, which might be applied to a flat surface of soil, not having the means of resorting to the more exact method of u->ing coffers in measuring the bases, and he accordingly sent me one, consisting of twenty links, each five feet long, of a simple but accurate construction.
An extract from Mr.
Trough ton's letter may not be uninteresting:—" The terminating lines " of the iron chain were set off from the British brass standard, at the temperature of 65" " of Farenheit's thermometer, this is 10° above the temperate point; the correction fbr " tliis, according to the expansion of iron with heat, is .07704
of an inch upon the whole " length, and those will be data for reducing that length for any other temperature; the " chain will, I think, require three people to carry it forward, for it should not be dragged " on the ground: care must be taken in folding it: in the most accurate operations the " pieces with sliding lines should be used for registering the end of the chain.
In other " cases the arrows may be sufficient, being mqre expeditious.
As the accuracy of the " chain is extreme, I leave you to judge if it may not supeisede the standard which you " ordered of me."
To convince myself of the accuracy and power of this instrument, I measured on the lake of Castlebar, when frozen, a distance of near two miles, and by repeating the measurement, it turned oiit, after making the necessary allowances for tern-peraiure, to come within one inch, a result I scarcely expected, and which proved its excellence for the purposes intended.
This trial was made by lines of coincidences, using the sliding pieces of brass for registering correctly the length of each chain, as the surface of the ice was perfectly smooth.
It was afterwards proved in several measurements on firm sands, diy at low water, and on level meadow-land, not to differ more lhan two inches ia two miles.
From the circumstance before stated, that Mayo afforded no flats, oa suffi-ciently favourable ground, of more than two miles in length, 1 determined on having a series of short and accurate bases, in preference to long ones, which might be doubtful, from having been measured over an irregular surface.
" .
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• .
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•*
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The following were the Bases measured':'-^
..
,
' -
'
-r«fc ;.
Base on the Moy plain flat meadow, subject to flood, in the barony of Gallan 6800 2.
Base on the plain at the head of Lough Carra, in the barony of Carra, meadow and bottom --_--_---_.,_-
633d"'* 3.
Base on the sands of Killala, in the barony of Tyrawley -9500 •"
, 4, Base on the sands between the island-of Annagh and Ini-hbegil, in the half n* barony of Erris
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og7--.sT'
I 2 5-
