(see plate 1.) in order to show us what part of the circle the four sides of the canister did really occupy; and as Mr. Speculative himself acknowledges in his preface to the experiment, that the rays of heat do proceed from the body in diverging lines; and these diverging lines must necessarily meet in the centre of the body, he would have directed us to draw the lines fe qr from the centre of the canister, and through the four corners of it, to the extremity of the circle. By this means he would have shown us, that the black surface of the canister exerted its power throughout the whole of that quarter of the circle marked No. 1. the clear surface in No. 2.

the paper surface in No. 3. and the glass surface in No. 4. He would next have directed us to draw the lines g h, from the centre of the canister to the extreme edges of the reflector. E; and would have made us observe that the space between these two lines occupied only 16 degrees of the circle; or scarcely one fifth-part of the horizontal rays which proceeded from the black surface of the canister. He would have told us, that, at the distance which the reflector was placed from the canister in this experiment, the horizontal rays proceeding from the black surface, would occupy a space equal to 5 feet and

a half horizontally, and as much vertically: that 5 and a half, multiplied by 5 and a half, would give us 30 square feet, as the space occupied by the rays proceeding from the black. surface of the canister; and, that as the reflector was only one foot in diameter, it could not receive one-thirtieth part of the rays proceeding from that side of the canister; and consequently, one-thirtieth part only, and not the whole as Dr. Speculative affirmed, could in this situation of the reflector, be concentrated upon the focal ball of the thermometer.

Dr. Speculative says, that having placed the canister with its black surface parallel to the reflector, every degree of obliquity which he gave to that side of the canister, in regard to the axis of the reflector, produced a corresponding effect, in reducing the coloured liquor in the thermometer. Mr. Truth, if he had been permitted to speak, would have said, that this also was not true; that Dr. Speculative had here concealed a part of the truth; and that he had in this experiment played off a small juggling trick, and brought -the clear surface of his canister into action, `in conjunction with the black one, in order to deceive us, and to give some semblance of truth to his own speculative opinion. He would have

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put us in mind, that he had told us in his part of the preface to the experiment, that, by giving the blackened surface of the canister a small degree of obliquity in regard to the axis of the reflector, the effect upon the thermometer was not visibly altered. He would have told us, that Dr. Speculative, in his account of this experiment, had endeavoued to conceal it from our knowledge, that no visible effect was actually produced upon the thermometer until he had given the blackened surface of the canister an obliquity equal to 37 degrees of the circle; and brought the corner a of the canister, and consequently the line e to correspond with the line g. He would have informed us, that this was the degree of obliquity which he alluded to in his part of the preface to the experiment. He would have told us, that Dr. Speculative still continuing to turn his canister round to the left hand, he then brought the clear surface of it to act upon the reflector along with the black one; that he thereby gradually diminished: the power of the black surface, and gradually increased the power of the clear one; and that that corresponding effect, which Dr. Speculative mentions in his account of the Experiment, was thereby produced. But,

If our friend Mr. Truth had been permitted to open his mouth upon this occasion, he would have told us, that Dr. Speculative, when he said, in his account of this Experiment, that the effect produced by the obliquity of the canister upon the thermometer, diminished; at first gradually, and afterwards with accelerating activity, was likewise not true: that the coloured liquor in the thermometer, did in that Experiment, descend gradually, and by a regular and uniform series of 5 degrees and a half, for every degree of the circle through which the corner a of the canister moved, in its passage from the line g, to the line h; where the black surface was thrown entirely out of action. He would have given us sensible and unquestionable evidence of this fact. He would for this purpose have recalled our attention to his first Experiment, where he showed us, that the power of the blackened surface of the canister raised the coloured liquor in the thermometer 100 degrees; and that when the clear surface of it was presented to the reflector, it raised it only 12 degrees. In order to discover the cause of that declension of the thermometer, which Dr. Speculative says was occasioned by the increasing obliquity of the blackened surface of the canis

ter, and which Mr. Truth has already proved, could only take place while the corner a of the canister was travelling round the circle from the line g, to the line h; he would have directed us to divide the numbers before mentioned, or the powers of each of these surfaces by 16, being the number of the degrees of the circle occupied by the reflector. That, by dividing the whole power of the black surface, viz, 100 by 16, we would have 6.25 or 6 and a fourth degrees of the thermometer, as the proportional declension of the power of the black surface; and in like manner, by dividing 12, or.120 in decimals, by 16, we would have .75 or three-fourths of a degree of the thermometer, as the proportional increasing power of the clear surface, corresponding to each degree of the circle, through which the corner a of the canister moved. Upon these clear and obvious facts, he would have directed us to construct the following table.

In the first column of this table, we have the declining power of the black surface; in the second column, the increasing power of the clear surface; and in the last column, the combined effect of both upon the thermometer, in every degree of the circle, through which the canister moves.

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