18 i REPORT—1843. 
which I selected for my tubes (0:1 inch) is not far from the truth. In the autumn 
of last year a fall of 1:6 inch produced no appreciable change of arc. The tempera- 
ture, however, was then nearly stationary ; but notwithstanding its changes during 
the interval from that time till my leaving Armagh, the are has been between 1° 36! 
and 1° 39’. Before the tubes were applied, the limits for the same period were 1° 42! 
and 1°51!. The changes in Bessel’s own clock, though made by Kessel, a first-rate 
artist, were still greater, being from 1° 25! to 1° 39’, an excess owing in part probably 
to the great severity of the German winter. From what I have seen of the vacuum 
apparatus used by Sabine or South, I cannot refrain from expressing a wish that the ex- 
perinient were tried of mounting a transit clock permanently in vacuo: such a clock 
would have many advantages, besides its exemption from changes of barometric 
pressure. 

Dr. Robinson communicated a Catalogue of Mean Places of 50 Telescopic Stars, 
within two degrees North Polar distance, observed at Markrea Castle, county of Sligo, 
by E. J. Cooper, Esq. 

On Contour Maps. By Captain Larcom. 
It is important that maps constructed by the government should exhibit the levels 
of the country in the most intelligible manner; showing heights not merely on the 
tops of hills, but round their sides, and through the valleys which traverse them. Such 
a system is offered by these contours. ‘hey are a series of horizontal lines, at a 
certain distance asunder, and at a certain height above a fixed datum. The datum 
most commonly used is the level of the sea, doubtless from the shore line being the 
limit of the land, and the point at which roads must cease, as well as from an impres- 
sion that it is itself a level line; and therefore, as the first contour, the most appro- 
priate and natural zero from which to reckon the others. The Section was aware 
that it has been a point much discussed, whether the high water, the low water, or 
the mean state of the tide, offers the most level line. This is a point which it would 
be out of place to discuss here, but it may be stated that, in order to determine it, as 
far as Ireland is concerned, a series of lines has been very accurately levelled across 
the island in various directions, and permanent marks are left in all the towns, and on 
numerous public buildings ; and at the end of each of these lines on the coast, tidal 
observations have been made every five minutes during two complete lunations, 
These observations, and the connecting lines of level, are now in process of reduction 
—the degree of accuracy attained is such that a discrepancy of *2 of an inch is im- 
mediately apparent—and from them we may expect many points of interest. The 
steeper the natural slope of the ground is, the closer together the contours of course 
will be, and the more oblique the road: where, on the contrary, the ground slopes 
very gently, the contours are further asunder, and the road may be proportionally more 
direct. By examining the maps of the Irish Survey, on which contours have been 
drawn, it will be seen that they tell sad tales of the existing roads, every one of which 
ascends and descends frequently, instead of keeping on a gradual slope for its whole 
length, In order to exhibit these lines, it is proposed, instead of adding them to the 
original copper-plate, which has a peculiar value as an official record of boundaries, 
to make a copy of the plate by the electrotype, for the purpose of receiving them, 
Contour maps were thought of early in the progress of the survey, but means were 
wanting for their execution; at present, however, the outline survey being complete, 
and the general map, or map of the surface, being in progress, a convenient opportu- 
nity is afforded, which it is hoped will not be lost, 


Account of the extraordinary Flux and Reflux of the Sea, July 5, 1843, at 
Arbroath. By Aurx. Brown. Communicated by Sir Davin BrewsTER. 
An extraordinary flux and reflux of the sea was observed at different places along 
the east coast of Scotland on the afternoon and evening of Wednesday the 5th of 
