Page:Transactions NZ Institute Volume 5.djvu/143

Rh which even isolated shingle of large size, or small boulders, have been carried has been about three miles, and they are still about fifteen miles from the lake, and with less chance of making even the same progress again, for the creek becomes flatter as it approaches the low grounds. It will be observed, also, that until the whole lake, up to its lowest water-level, has been filled by solid material, its utility for storing flood waters is unimpaired. I am not in a position to state the contents to that level, but taking its depth at five feet— which I believe to be within the mark—it will be granted, I think, that even with much increased diggings it is safe for many years. If not, then the sooner the outlet is raised artificially the better.

But a real argument for a portion, at least, of this work is to be found in the neighbourhood—from the fact that there are now two outlets from the lake proper, while, before the 1868 floods, there was only one; and also the narrow gorge at the foot-bridge was widened by about an eighth part in the flood of 1870, and from the nature of the strata—being basalt, with very many joints, overlying clay—it is liable to greater extension, and, consequently, to allow the water to come more quickly towards the lower parts and facilitate floods. If it should so happen, in succeeding floods, that the same enlargement of these three outlets should continue, the utility of the lake as a regulating reservoir will be very much reduced; and the more rapid delivery of its waters may almost enable a flood equally as destructive as that of February, 1868, to result from less rain.



we take a small plane mirror and reflect a parallel beam of light from any distant luminous object, as the sun, on to any fixed point, and then arrange another small mirror close to the side of it, so as to reflect the light from the sun to the same point as the first mirror, and thus proceed to any extent, arranging a number of such mirrors in one plane, so that they shall all reflect the incident ray to the same point, (Fig. 1. F), then because the angle of incidence of a ray of light is equal to the angle of reflection, the curved line joining the centres of these mirrors forms the arc of a parabola, and each mirror when so arranged is a tangent to this arc, the surface generated by the revolution of such an arc on its axis being termed a paraboloid. 