BURNING MIRRORS.
129
the focus, and what it really was, of necessity points out theerrors in the instrument, which must arise from one of twocauses, or, perhaps, from both; i.e. that the axis of thetwo sides are not coincident,—or that they are not oppositeto each other; should the difference be occasioned bv either,the focus would have been elliptical; but this not being 1ascertained, it can be only said that a very small deviationin each would have occasioned this enlargement of the focus;and it is possible they might have acted in contrary ways,which would have made it nearly circular.
As this ingenious lens was exported many years ago,before it was ascertained to what cause the above mentionedvariation was ascribable, i. e. whether the focus stated tohave been one inch in diameter, was really ascertained byits burning effect, or by the diameter of the focal rays, theenlargement whereof must have arisen from the previouslymentioned causes; but if it could be ascertained from itsburning power, it assuredly tends to confirm Dr. Her&clieFsopinion, that the rays of caloric are less refrangible thanrays of light, and which, bv-the-bye, also confirms a previousopinion on the doctrine of light, we have presumed tohazard. And then the true burning focus will be discoveredto be rather beyond the apparent focus. The ingeniousexperiments of Mr. Wedgwood, and the extensive use hisPyrometer has obtained, for the admeasurement of heatrequired to melt metals, are now so well established, andtoo well understood to render eulogium necessary; it maybe sufficient to state his estimates of the heat required
to melt the following substances.
Fahrenheit. Wedgwood.
Red heat, visible in day-light. 1077 3 0 D
Fine gold. 5237 32
Welding heat of iron, greatest .... . 13,427 95
Cast iron melts. 17,977 130
Greatest heat of his small furnace.. 21,877 160
Extremity of scale of pyrometer .. 32,277 240
The power of the lens, estimated by this pyrometer, willbe found to be 1096 J ; aud presuming the hottest furnace tobe 240, the focal heat thereof is 5.2975 times greater.
The learned and ingenious author, from whom these ex-tracts are taken, then, in justification of his previously-citedopinion, proposed to submit a selected enumeration ofexperiments made with that celebrated instrument, underthe inspection of several scientific persons, and also mem-bers of the Roval Society, and others.
VOL. II.
K