8 Biograplikal Memoir of Sir John Leslie. 



the proposal could not but prove agreeable to him, and it was 

 accordingly embraced with alacrity. The tour was to com- 

 mence eai'ly in 1793 ; but a change of plan having been decided 

 upon without his concurrence, and by which he found that his 

 observations were to be transferred to another, he immediately 

 relinquished his engagement. His intention to visit these in- 

 teresting countries, when a fit opportunity should occur, re- 

 mained with him through life ; and, at a period nearly forty 

 years subsequent to this, — in his last years, indeed, — he actu- 

 ally made some pi'eparations for a year s sojourn in Egypt and 

 Palestine, from which he was diverted only by engagements 

 and avocations of which he found it difficult to disencumber 

 himself. 



After leaving Etruria, he passed some months in Holland ; 

 a coiuitry which, like Descartes, he seems to have thought pecu- 

 liarly suited to secluded study, and where he at this time ac- 

 quired the German language. Thereafter, he returned to Largo, 

 where he remained for about two years, devoted to experimen-' 

 tal researches ; in the course of which he invented and perfected 

 his Differential Thermometer ; the parent, if we may so speak, 

 of that beautiful family of Philosophical Instruments,* Avith 

 Avhich he enriched the Treasury of Science, and amplified and 

 variegated the means of physical inquiry. His ingenuity had 

 been early exercised in some attempts to construct an accurate 

 Hygrometer, and these ultimately suggested to him the well- 

 known contrivance above named ; — a contrivance happily adapt- 

 ed to the measurement of the smallest variations of temperature, 

 and which richly rewarded his inventive powers by its ministry 

 to the achievement of his subsequent discoveries. It has gene- 

 rally, indeed, been allowed to be one of the most useful as well 

 as elegant inventions that inductive genius ever applied to the 

 investigation of chemical changes. At a later period, when his 



• These instruments, viz. the Differential Thermometer, Hygrometer, 

 Hygroscope, Photometer, Pyroscope, yEthrioscope, and Atmometer, are all 

 described, and their principles explained, by himself, in the articles[Climate and 

 Meteorology in this Encyclopsedia. Properly speaking, the Hygrometer was the 

 parent instrument ; the Differential Thermometer hav'ing been invented in the 

 course of his endeavours to improve it; but as the Hygrometer, in its latest 

 form, is only a modification of the other,it may be represented as derived from it. 



