BURNING MIRRORS.

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substance. It was double-convex, both of its sides were aportion of a sphere of 18 feet radius. The largest lens,whose diameter in the frame was 3 feet, in the clear2 feet 8J inches; its thickness at the centre, 31 inches; itsfocal distance (i feet 8 inches. Diameter of the burningfocus at that distance 1 inch; weight 212 lbs. It had asecond lens, whose diameter in the frame was 16 inches,showing in the clear 13 inches; thickness in the centre 1£of an inch.; its weight 21 lbs. When both lenses were com-pounded, the length of the focus was 5 feet 3 inches; itsdiameter half an inch. Doth these lenses were fixed in atruncated cone, formed of twelve ribs of wood : at the largerend Was fixed the great lens, and in the small extremity thelesser lens, where is also fixed a rack passing through apillar, moveable by a pinion turning in the said pillar, bymeans of a handle, and thus giving a vertical motion to themachine. A bar also of wood was fixed between the twolower ribs of the machine, which moved an apparatus withan iron plate fixed to it, which, at this part, turned on asocket, thereby affording a method of placing the matterunder experiment, so as that it may be acted upon by thefocal rays in the most direct and potent manner. It isplaced in a strong mahogany frame, moving on castors; im-mediately under the table of it are three friction wheels, bywhich the machine is moved horizontally. There is also astrong iron bow, on wdiich the lens and cone hang. If it bedifficult to furnish an idea of this piece of ingenious mecha-nism without plates, it becomes much more so to give posi-tive testimony as to its effects. Because, it should appear,that circumstances attended that insti ment, which dis-tance human calculation; as it cannot be ascertained whatshould be deducted for the loss of power in consequenceof the impediments in the glass of which it was made,as wel 1 as from the four reflections, that is, a doublereflection on immersion, and two more on emersion of therays of heat, but by way of diminution;—a data has beenfixed to appreciate it, by making a full allowance for thosedeductions which must be considered as the result of a con-centration of the power of the solar rays, after deductingample allowance for subtraction, in consequence of the pre-viously surmised obstructions, analogous to those usuallyallowed for the friction of an engine, of which nature theyhave been conceived to partake.

The solar rays received on a circular surface of 2 feet 81inches, w'hen concentrated within the diameter of oneinch, will be 1056.25 times its intensity, or this numberof times greater than the heat of the sun, as falling on the