It was about the year 1794.95, while occupied upon a long series of hygrometrical experiments, that be either re-invented or borrowed the Collegium Experimental° ' of Sturmius his Differential Thermometer. He supposed the propagation of radiant heat to take place by means of aerial pulsations, a supposition which appears irreconcileable with the existence of radiation in vacuo, and equally at variance with the more recent experimental results of Messrs. Duloog and Petit. He assumed moreover the universality of what is usually termed Newton's law, namely, "that the decrements of heat of a cooling body are proportional to the difference between its tempe rature and that of the surrounding medium ;" whereas it is known to hold only so hang as that difference does not exceed from 40° to 50°.
His own theories indeed sometimes appear to be rather the effusions of a bold and active fancy than the logical deductions from any established facts, and, as an almost inevitable consequence, the results to which they lead him appear equally fanciful. Of this character are his conclusions, that "the matter of the moon is phosphorescent, and at some future period our satellite will become dim and seem blotted from the blue vault of heaven ;" that "the earth contains a concavity filled with concentrated light, shining with intense refulgence and overpowering splendour," and others of like nature. He regarded the inventive faculty as the highest with which the mind can be endowed, and attached so little importance to inductive philosophy that he has been heard to deny that any merit is duo to Bacon as its founder. As an author, he was deficient in systematic arrangement and simplicity of style. As a lecturer, he was liable to fall short of a satisfactory elucidation of his subject by estimating too highly either the capacity or the previous knowledge of his auditors. But on the other hand, his active curiosity, varied reading, and powerful memory, led to the acquisition of very extensive knowledge, which in many instances he successfully applied to the promotion of science, and "his exquisite instruments and experimental devices will ever attest the utility no less than the originality of his labours."
Besides the works noticed in the preceding part of this article, he has left ' Elements of Geometry, Geometrical Analysis, and Plane Trigo nometry,' 8vo, 1809; the same abridged, 1828 ; Geometry of Curve Lines, 8vo, 1821; 'Philosophy of Arithmetic, 1817; Account of Experiments and Instruments depending on the relations of Air to Heat and Moisture,' 12rno, 1813; Elements of Natural Philosophy,' vol. i. (containing Mechanics and Hydrostatics), 8vo, 1823.
In the Edinburgh Philosophical Transactions :—` Observations on Electrical Theories,' 1824 ; certain Impressions of Cold mitted from the higher Atmospheres, with a Description of nu Instrument adapted to measure them,' 1818.
In the Encyclopcedia Britannica :—Articles 'Achromatic Glasses;' 'Acoustics;' ' Aeronautics;" Andes ;" `Angle;' `Angle, Trisection of;' Arithmetic ;' Atmometer ;" Barometer ; "Barometrical Mea surements; "Climate ;" Cold and Congelation ; " Dew ;" Inter polation ;" Progress of the Mathematical and Physical Sciences during the Eighteenth Century.' • In the Edinburgh Review :—Papers on the 'Memoirs of the Society of Arcuoil ;' on the 'History of the Barometer ;' on 'Delambre s Arithmetic of the Greeks ;' on Vou Buch's ' on Humboldt's Physical View of the Equatorial Regions,' and his 'Travels;' on the to discover a North-West Passage.' In Nicholson's Philosophical Journal, vols. iii. and iv. :—'Description of an Hygrometer and Photometer;' ' On the Absorbent Powers of different Earths ;"Observations on Light and Heat, with Remarks on the Enquiries of Dr. Herschel' Some papers by him on physical subjects were also read before the Royal Society of London, but none were ever printed in their Transactions.' (Memoir of Sir John Leslie, by Macvey Napier, 1838.)