from the following simple illustration. From the station of Culeagh, the Mayo mountains were visible over the intervening Leitrim ridge, and the double peak of a small mountain was a very remarkable object. Now, from the peculiarity of its form, this mountain might be observed, according to the refraction, sometimes as a single conical peak, and sometimes as having two peaks, one being higher than the other, whilst on other occasions it was cut off entirely from the observer's station by the intervening ridge. Another system was therefore adopted initial bases of altitude were fixed with great care along the eastern and western shores, and observing points were selected, running in lines directly from the one side of the island to the other. These points were distant from each other from 8 to 10 miles, and reciprocal observations were taken between each successive pair; but as a further check, a subsidiary station was selected between each pair of the observing stations, and was observed from the stations east and west of it. By this arrangement, the difference of altitude deduced from the reciprocal observations of the observing stations, which might have been affected by some change in the amount of refraction of one or the other, was brought into comparison with the same difference as deduced from observations made to the intermediate or subsidiary station which being comparatively so near to each of the pair of stations, would be very little affected by any small variation in the refractions. The first line of altitude observations of this

description, was adopted by the author to connect the heights of the Antrim and Londonderry surveys, and to reduce the junction points of adjacent districts to harmony. It ran obliquely from the eastern coast of Antrim to the northern coast of Derry, and was most effectual in giving to the altitudes of those counties a high degree of accuracy, although the observations were taken with an instrument of less size, and very difficult of manipulation. The author afterwards carried several similar lines across the country, using the great theodolite as the observing instrument; and it may fairly be said that the altitudes were excellent, although affected by the error consequent on the use of the low water line of spring tides as the zero of altitude. This had been the referring line in the British survey, but General Colby, who had encouraged every step taken to improve the altitudes, readily abandoned it for the more correct mean tidal line, and perfected the survey altitudes, by having other lines of altitudes determined by careful levelling with the spirit level. In every survey, these two modes should be used in conjunction with each other, lines of initial altitudes being first determined by the spirit level, and these lines connected together by traversing lines of altitudes, determined in the way which has been explained.

The primary and secondary altitudes being thus reduced to a uniform standard of accuracy, the subsidiary and chain line altitudes became also safe points of reference for practical operations of drain

age, of road making, &c.; and it is at this point that some would have had the progress of improvement stop but General Colby was not blind to the advancement of topographical science on the Continent, and was ready, therefore, to introduce into the British survey and map any of the improvements which had been adopted there. Looking then at the early sheets of the Irish map, the engineer will be struck by the vast amount of data expressed upon them in regard to altitudes, and may also trace, in many cases, the gradual rise of a hill, by following the course of a chain line, and noticing the successive levels marked along it; but in others, he will find it a painful task to unravel the confused web of levels scattered over the map, or to deduce from them any notion of the features of the country. In a map of moderate size, such as the one inch, in England, some description of shading would of course assist in reducing levels to order, or would even supply their place; and to this object General Colby gave much attention at the commencement of the Irish survey, visiting all the mountain chains, and after a careful consideration of the features of the country, ordering a general map to be drawn, so as to afford a graduated scale of shading to be used subsequently in the inch map, the necessity and preparation for which were never absent from his mind. To a map however on so large a scale as the 6-inch, ordinary shading was inapplicable, and General Colby therefore, in the first instance, left the reader of the map dependent solely on the engraved altitudes. In

1837 and 1838, however, the data of the survey were called into practical use, in aid of another great public object, namely, the determination of the best lines of railway for Ireland; as in that country, so often underrated and even contemned in regard to its supposed want of utilitarian habits, the wise example was set to its more confident sister country, of making the railways parts of one general system, and not a set of isolated and disconnected systems. Commissioners were therefore appointed, using the statement of Lieutenant (now Lieutenant-Colonel) Larcom, "in 1837 and 1838, to report on the most advantageous lines for railways in Ireland, and other matters connected therewith: a general map of the kingdom was deemed necessary, and Lieutenant Larcom was entrusted with the preparation of such a map. On this map the various lines of railway proposed by private parties, as well as those recommended by the Commissioners, were to be laid down, and the map was also to be the means of exhibiting, for the first time, the geology of the whole island, by Mr. Griffiths. The Ordnance survey had extended over only one-third of the country, but the triangulation was complete for the whole island. the aid of this, the principal latitudes and longitudes were computed, and a net work prepared, by which the detail of the Ordnance survey and the various local surveys some published, many fragmentary and in MS.-were connected. An outline map was thus completed, but, for geological purposes it was also necessary that the mountain ranges and features

By

of the ground should be delineated. For this purpose, such material as existed was collected and combined, after which persons practised in that branch of topography were despatched to the principal localities either where the information was most defective, or where the greatest accuracy was required. From the drawings and enlarged sketches of these parties the original work was corrected, and ultimately a map on the scale of 4 miles to the inch was drawn and engraved. It is believed that this was the first general map of any country in Europe, on which the ground was completely delineated as one whole; and as yet it is the only such map of Ireland. Made, as it was, in much haste, and from very imperfect material, its accuracy could be only comparative, being limited, of course, to that which the original material possessed, with such aid from compilation as careful collation and comparison could supply, with the valuable materials of the Ordnance triangulation for its general basis. It was deemed satisfactory, and the Commissioners by a letter, forwarded through the Treasury, to the Master General of the Ordnance, dated 18th March, 1839, acknowledged the assistance it had afforded them."

CONTOURS.-"It was in connection with this map that Lieutenant Larcom was led to consider the importance and the practicability of adopting the continental system of contours, for exhibiting the third co-ordinate of topographic delineation-the altitude. At his instance, experiments were tried, and an officer, Lieutenant Bennett, set apart for