1 48 Decisions on Disputed Invention.':. 



4. Professor Leslie's Hygrometer, invented by the late Dr James Hullon. 



An account of Mr Leslie's hygrometer was first published in Nicholson's 

 Philosophical 'Journal, 4to, vol. iii. p. 461, and it appears from that de- 

 scription, that the principle of the instrument, as well as the construction 

 and application of it, are claimed as the sole discovery of the author, as the 

 following quotations will show : — 



" My attention was first directed to the subject of hygrometry by the 

 perusal of the late Dr Hutton's very ingenious Theory of Ruin." — ' To 

 discover the dryness or humidity of the air, we have only to find the 

 change of temperature induced on a body of water insulated or exposed on 

 all sides to evaporation. This principle I first established in 1790. 



" Two thermometers, therefore, filled with any expansible fluid, with 

 quicksilver, alcohol, or air, the hall of the one being wetted and the other 

 dry, will, by their difference, denote the state of the air in respect to humi- 

 dity. Nothing was wanted but to combine those instruments in such a 

 manner as that they should indicate their differences of temperature." 



It appears, from these passages, that Mr Leslie considers himself as having 

 discovered and established the principle of the hygrometer in 1790; and 

 yet he regards this principle as carried into effect by the use of two ther- 

 mometers, so contrived as to indicate merely their difference of tempera- 

 ture. 



It appears from Professor Playfair's life of Dr Hutton, in the Edinburgh 

 Tiunsactions, vol. v. pp. 67 and 106, that such an instrument had been 

 invented and used by Dr Hutton himself, though he had not described it 

 in any of his publications. The following is the passnge : — 



" To one," says Professor Playfair, " who considers meteorology with 

 attention, the want of an accurate hygrometer can never fail to be a subject 

 of regret. The way of supplying this deficiency, which Dr Hutton prac- 

 tised, was by moistening the ball of a thermometer, and measuring the de- 

 gree of cold produced by the evaporation of the moisture. The degree of 

 cold, caeteris paribus, will be proportional to the dryness of the air, and 

 affords, of course, a measure of that dryness." 



It is very obvious that, by means of a double thermometer and a move- 

 able scale, the difference of the indication of the two thermometers may 

 be observed at once- Mr Leslie applied the same principle to Sturmius's 

 differential thermometer; and, consequently, he has done nothing more 

 than use an air thermometer in place of a mercurial one, though it does 

 not appear from Mr Phyfair's account what kind of thermometer was em- 

 ployed by Dr Hutton. 



We believe it is now universally admitted, that the hygrometer, as manu- 

 factured by Mr Leslie, is not an accurate instrument, and we know that 

 it is not in repute in France, and has not been used by any of the emi- 

 nent French philosophers who have carried on hygrometrical researches. 

 They have invariably preferred the hair hygrometer of Saussure. 



Those who prefer the employment of Dr Hutton's principle ought to 

 use the two thermometers in place of the differential one, as the tempera- 



