94 PLAYFAIR. 



written with all the perspicuity, caution, and sagacity, that con- 

 stitute the great excellence and the great difficulty of such disqui- 

 sitions, where scientific principles are employed to give precision to 

 physical observations. In 1790 appeared, in the same ' Transactions,' 

 a paper of still greater interest and delicacy, ' On the Astronomy of 

 the Brahmins,' the publication of which attracted very general 

 attention, both in Europe and in Asia, and gave rise to much dis- 

 cussion and research. This was followed in 1794 by a learned and 

 very beautiful treatise on the l Origin and Investigation of Porisms,' 

 in which the obscure nature of the very comprehensive and indefi- 

 nite theorems to which this name was applied by the ancient 

 geometers, is explained with the most lucid simplicity. 



In 1797 he composed a sequel to his first paper on the Indian 

 astronomy, in the shape of ' Observations on the Trigonometrical 

 Tables of the Brahmins,' and also a masterly collection of ' Theorems 

 on the Figure of the Earth.' During the course of the last-men- 

 tioned year, his friend Dr. James Hutton died ; and Playfair, having 

 undertaken to draw up a biographical account of him for the Royal 

 Society, was led to study the doctor's ingenious but crude specula- 

 tions on the ' Theory of the Earth,' and afterwards to lend them the 

 assistance of his own powerful pen, in his * Illustrations of the Hut- 

 tonian Theory.' This work, upon which he bestowed more time 

 and labour than on any of his other productions, did not appear 

 until 1802 ; and whatever opinion may be formed of the truth or 

 soundness of Dr. Button's speculations, it is impossible to doubt 

 that Playfair's illustration of that theory must always be ranked 

 amongst the most brilliant and powerful productions of philoso- 

 phical genius. Its merits have been universally acknowledged, even 

 by those not convinced by its reasonings, and have extorted, even 

 from the fastidious critics of France, the acknowledgment that 

 " Mr. Playfair writes as well as BufFon, and reasons incomparably 

 better." 



In 1805 he quitted the chair of mathematics to succeed Professor 

 Robison in that of natural philosophy. In the contest which ensued 

 upon the appointment of Leslie as his successor in this chair, he 

 took a very active part ; and amongst the heaviest blows which 

 Leslie's opponents had to sustain, were those that parted from the 

 hand of Mr. Playfair. In 1807 he was elected a Fellow of the Royal 

 Society, to which learned body he very soon afterwards presented 

 his ' Account of the Lithological Survey of Schehallien ; ' this was 

 the result of his investigations during the period of Dr. Maskelyne's 

 visit to Schehallien, to measure the attraction of that mountain, on 

 which occasion Playfair shared the shelter of this distinguished 

 astronomer's tent on the side of the mountain, and contracted with 

 him a friendship which lasted during the remainder of their lives. 



In 1809 he contributed to the * Edinburgh Transactions' an ex- 

 cellent paper on ' Solids of the Greatest Attraction,' and in 1812, 



