In prosecuting a trigonometrical survey, the altitudes of the principal mountains, &c. are noted, due allowance being made for the curvature of the earth, and for the effects of refraction, which latter affects the apparent altitudes of both celestial and terrestrial objects. This subject shall be fully explained when we come to the portion of the work which treats of levelling.

Observations of Triangulation.

After what has been said on the subject of choosing proper stations, in the part of the work devoted to parish surveying, and the surveying of estates trigonometrically, it is scarcely necessary to mention, that in extensive trigonometrical surveys, the triangles should be chosen as nearly equilateral as the nature of the country would admit.

The base line being very small in comparison with the great triangles to be subsequently deduced from it, every successive triangle should increase as rapidly as possible, consistently with the nearly equilateral form of each triangle.

The length of the base line of the Ordnance Survey of Ireland did not exceed ten miles, though some of the subsequent lines exceeded one hundred miles. Το deduce such large triangles from the original one, which was comparatively diminutive, without the introduction of an ill-conditioned one, required considerable judgment

If the face of the country permitted it, the following method, proposed by Lieut. Frome, R.E., of enlarging

Let AB

the subsequent triangles, is an excellent one. be the base line; at A and B measure the angles CAB,

DAB, CBA, DBA; hence CD may be found. Next, the angles ECD, FCD, EDC, FDC are measured, from which, with the computed line CD, the length of the line EF may be computed. In like manner, by measuring the angles at the points E and F, the line GH may be computed. By such a process the triangles must soon arrive at the greatest limit at which the station point can be rendered visible. Colonel Colby and Captain Kater employed a plane mirror to render distant objects visible. It was by this means they verified General Roy's triangulation for connecting the meridian of Greenwich and Paris.

Captain Drummond, R.E., our late under Secretary of State for Ireland, having been aware of the utility of thus employing the reflected rays of the sun, invented an instrument, by means of which the solar rays were directed towards the station to be rendered visible.

A description of this instrument may be seen in a paper written by himself, and published in the Philosophical Transactions for the year 1826.

This Heliostat being useful only when the sun shone, another contrivance was still wanting to produce a light sufficiently strong to render distant stations visible at night. General Roy employed Bengal lights for this purpose, which were superceded by argand lamps and parabolic reflectors, which rendered objects visible at the distance of forty-eight miles. But the light invented by the late Captain Drummond, above alluded to, surpasses them all in intensity of brilliancy. This invention consists in a stream of oxygen gas directed through a flame of alcohol to a ball of lime, about a quarter of an inch in diameter, placed in the focus of a parabolic reflector. This produced light eighty times as intense as that produced by Argand's lamp. Any one having seen this light must be at once convinced of its fitness for light-houses where the strongest illumination is required.

The superior power of the Drummond light was proved in a special manner in the neighbourhood of Belfast, where, in hazy weather, it was brilliantly visible at the distance of sixty-seven miles.

The author has had opportunities of seeing all the instruments used by Colonel Colby in the Ordnance Survey of Ireland, and has been particularly struck with the beautiful contrivance resorted to for preventing any expansion or contraction between the points marked at the extremities of the metal rods used to measure the base line.

Two parallel bars, the one of iron and the other of

brass, each ten feet long, are rivetted together at their centres. The brass bar is coated with some non-conducting substance, to equalize the susceptibility of the metals to change of temperature.

The extremity of each bar is furnished with a tongue of iron, having a minute dot of platinum, and so placed on this tongue, that under every degree of expansion or contraction of the rods, the dots at each end remained at the constant distance of ten feet. There are accompanying microscopes attached to the end of similar compound bars, six inches long. The microscopes in these short bars occupy the position of the dots in the longer bars. After repeated experiments, to ascertain the relative expansion and contraction of these bars, in their transitions from cold to heat, and from heat to cold, they were found to be in the proportion of three to five.

This invention of Colonel Colby's is more expeditious in its practical application than any other method employed before for the measurement of a base line. In the methods usually employed for this purpose, the temperature is noted as every chain or rod is laid, and an allowance made for the contraction or expansion occasioned by every change in the thermometor.

What has been given here is but a faint outline of Colonel Colby's invention, a full account of which, it is hoped, will soon be published by himself, with the other details of the Ordnance Survey of Ireland.

When a base line is measured on an elevated plain, it must be reduced to its proper measure at the level of the sea; and though this correction is in general but very small, yet in the measurement of a base line, upon