H. Skey. — An Astronomical Tdescope on a New Construction. 119 



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 ive 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 enlarge- 

 ment 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. 



AiiT. XI. — An Astronomical Telescope on a New Construction. 



By H. Skey. 



(With Illustrations.) 



[Read before the Otago Institute, \^th November, 1872.] 



If 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. 



