MR. BUNT'S DESCRIFnON OF A NEW TIDE-GAUGE. 251 



on the cylinder. Successive marks, as the tide and pencil continue to ascend, at 

 length complete the scale. A better and more expeditious method is by accurately 

 measuring and dividing the circumference of the large float wheel, and then marking 

 the cylinder vrith the pencil as it is wound up, by moving the wheel round and bring- 

 ing its divisions successively up to a fixed index. 



The pencil curves may be either received on the enamelled surface of the cylinder, 

 or they may be taken off on paper, very correctly, in the following manner. The 

 sheet having been wrapped round the cylinder, and expanded by rubbing it all over 

 with a wet sponge, is then pasted together at the ends, and secured in its place, either 

 with drawing pins, or with elastic bands, as shown in the drawing ; after which it 

 soon becomes quite tight, and the curves may be taken and read off on the paper with 

 nearly as much accuracy as on the enamel itself, with the additional advantage of 

 their being preserved. 



The times and heights are read off by means of the vertical scale M, which is held 

 at the two extremities by notches cut in the frame, and may be pushed into contact 

 with the cylinder. The large wheel being detached from the clock, and a short chord- 

 line drawn under the summit of each of the curves and bisected, the point of bisection 

 is turned round to the edge of the scale, the upper end of which shows the time of 

 high water, amongst the divisions of hours and minutes on the cylinder. The height 

 is read off by noticing where the summit of the curve falls amongst the graduations 

 of the scale. Thus in fig. 2. the curve in contact with the scale indicates the time 

 of high water to be 0*^ 40™ p.m. and the height 32 feet 11 inches. 



The float is of pine, well saturated with oil, and has hitherto preserved suflScient 

 buoyancy. It is without friction rollers, which it is better to dispense with when its 

 motion is very nearly vertical. The trunk is of cast iron Jths of an inch in thickness, 

 being in a situation where considerable strength was required. At the back of the 

 trunk is a stout wooden plank, and both together are firmly secured with nuts, and 

 bolts inserted with lead into the quay-wall. The joints are caulked, and the water 

 enters through a moveable shutter in the bottom, by an aperture of about ir^th of 

 the sectional area of the trunk. This was found after many trials to be the best size 

 for preventing unsteady motion of the float, which is now almost entirely removed ; 

 a rapid undulation being communicated to it when the aperture was made much 

 larger, and a slow but very considerable oscillation when it was much less. 



2 k2 



