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394 Proceedings of the British Association. 
by swinging the pendulum in a vacuum apparatus, in 1829. The 
amount of it is far from inconsiderable ; even with the mercurial pen- 
dulum of a transit clock, weighing 21 pounds, and presenting a very 
small surface, it is 0°36 for an inch change of the barometer. The 
remedy is obvious: by attaching a barometer to the pendulum, its fall 
transfers a cylinder of mercury from a point near the axis of motion 
to a greater distance from it; the time of vibration may thus be made 
to increase by the same amount that it decreases in consequence of the 
diminished density of the air. By placing the clock in vacuo as Bessel, 4 
proposes, (and as Sir James South has actually done for several years 
past,) the effect of resistance can be determined exactly, and ‘the diame- 
ter of tube selected, which will nearly correct it. ‘The diameter select: 
ed by Dr. R. (01 inch) is not far from the truth. In the autumn of last 
year, when the temperature was. nearly stationary, a fall of 1-6 inch 
produced no appreciable change of arc. 
“‘On Contoured Maps,” by Captain Larcom. It is important that 
governmental maps should exhibit the levels of the country in the most 
- intelligible manner; showing heights not merely on the tops of hills, 
but around their sides, and through the valleys which traverse them. 
Such a system is offered by these contours. They are a series of hor- 
izontal 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 impression that it 
is itself a level line; and therefore, as the first contour, the most ap- 
propriate and natural zero, from which to reckon the others. It has 
been a point much discussed, whether the high water, the low water, OF 
the mean state of the tide, offers the most level line. Capt. L. stat 
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 di- 
rections, and permanent marks are left in all the towns, and on numer 
ous public buildings; and at the end of each of these lines on the coast, 
tidal observations have been made every five minutes during two com. 
plete 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 immediately apparent—® 
from them we may expect many points of interest. The steeper the 
natural slope of the ground is, the closer together, of course, the con- 
tours will be, and the more oblique the road; where, on the contrary» 
the ground slopes very gently, the contours are farther asunder, and the 
road may be proportionally more direct. “¢ Te SS 
The Rev. Prof. Lloyd read a paper, by Rev. T. Knox, “on the Quan 
tily of Rain which falls in the S. W. of Ireland, and in Suffolk, Eng 
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