It will be observed, that the degrees of the circle in the left hand column, commences in the line g, where the black surface of the canister continues to act with its full power; and where the clear surface has not yet come into action and that it ends in the line h, where the power of the black surface entirely ceases, and where the clear surface acquires its full power; comprehending 16 degrees of the circle.

In this way Mr. Truth, if he had been permitted to open his mouth in the course of this experiment, would have shown us that what, he said in the preface to the experiment was true, and that the rays of heat, proceeding from a heated body, are most unquestionably equally diffused through the adjoining space.

"

Dr. Speculative, however, draws a very opposite conclusion from this deceptive experiment which he had contrived. He says, "The "impressions of heat or cold are, therefore, "propagated through the air with unequal de

grees of diffusion. Their force is evidently 66 greatest in the line perpendicular to the sur"face, and regularly decreases as the direction "becomes oblique. Between that force and the "angle of obliquity some relation must subsist; which is the next object of inquiry." Dr. Speculative now proceeds to

He says,

EXPERIMENT XXI.

"Provide a tin screen composed of "two sliding parts that shut together in a vertical "line,but leave, when opened, an aperture or slit "of any required breadth. Arrange the apparatus "as formerly, and plant the screen a little before "the canister, and parallel to its blackened side.

Open the screen by drawing out both sides "equally; and note the effect produced upon "the differential thermometer. Now turn the "canister about upon its centre, till it is just "sufficient to fill up the void space behind, or "such, that no straight line can pass by the edge "of the canister, and through the aperture, "to the reflector. As the As the aperture is thus suc❝cessively contracted, the canister will acquire “an inclination always more oblique."

This and the two succeeding Experiments, are mere repetitions of the last one. This tin sliding screen seems to be the last effort of Dr. Speculative's genius, for the purpose of throwing a shade of mystery over his experiments; vainly imagining, that he would be thereby able to deceive the penetrating eyes of Truth, who still attends him, and shows us all the vile artifices which he employs to deceive us. It is perfectly obvious, that this screen is a mere deception, and could be of no sort of use in the experiment.

For, if the aperture in the middle of the screen was equal to the space between the lines g and h, (see plate 1.) all the rays of heat proceeding from the canister, would pass as freely to the reflector, as if the screen had not been there; and

the effect would continue the same, whatever degree of obliquity he might give to the canister, until he brought the corner a to correspond with the line g, as was shown in the last experi ment. If he made the aperture less than the space between the lines g and h, he would then obstruct a part of the rays in their in their passage to the reflector; and he might in this way reduce the effect upon the thermometer in any proportion he pleased. But could this be considered as any proof that the rays of heat are not equally diffused? These remarks are fully confirmed by our friend Truth, in the observations which he makes upon this experiment. He says,

"In every case the impression made upon the "focal ball will depend upon the quantity of

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aperture, and will be nearly the same, whether "the canister stands parallel or inclined to the "screen. When the obliquity becomes indeed

very considerable, a small diminution of effect, "seldom amounting to the tenth or twentieth part begins to be perceived."

..

Does not our friend Truth here confirm what he told us in the preface to the 20th experiment where he says That the obliquity of the canister produced no sensible difference in the effect? And does he not as directly contradict what Dr. Spe

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