Mr. Howarp Gruss on Great Telescopes of the Future. 11 
_ The lower story, D, is constructed with a large fire area and a set of nearly semi- 
circular flues, any one of which can be used or stopped at pleasure, and thus a 
means is given of modifying the heat of any portion of the floor of upper oven in 
which the speculum or glass 1s placed. 
In the second story, C, is placed the speculum or glass, and lying on it are the 
extremities of 10 thermo-electric couples. These are supposed to be all rated 
beforehand, so as to give equal deviations of the magnetic needle for equal tempera- 
tures, or if not, their scales should be known. 
One of these couples is placed lying on centre of mirror, the others are arranged 
in three sets, 120° apart, and at different distances from the centre, so that the mean 
of reading of three couples equally distant from centre will give a fair estimate of 
the relative heat of that zone of mirror to the other zones thus—A.A’A” : B.B’.B’ : 
C.C’.C” : D—will give the proportional heats of the 1st or outer zone to the 2nd, 3rd, 
and centre. 
The relative rates of cooling of different parts being thus known, the heat can be 
modified by allowing heated air or cold air to pass through the various flues. 
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asc D.C’ A’,c’ pd” 
I mentioned above that I would speak subsequently respecting a method of 
