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 3 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 

 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. (0-1 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, or 

 the mean state of the tide, offers the most level line. Capt. L. 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 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 — and 

 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. 



The Rev. Prof. Lloyd read a paper, by Rev. T. Knox, '■'■on the Quan- 

 tity of Rain which falls in the S. W. of Ireland, and in Suffolk, Eng- 



