History of' the Differential Thermometer. 145 



ences of temperature ; and he made it the basis of a series of instru- 

 ments which bear his own name, and which have been manufactured and 

 sold by himself. 



About the same time, or perhaps earlier,* Count Rumford announced 

 the construction of an instrument, called a Thermoscope, which he used 

 in the same manner, and for the seme purposes, and which, he says, " he 

 contrived for measuring, or rather for discovering, very small changes of 

 temperature." Count Rumford was loudly charged, not only with hav- 

 ing stolen Mr. Leslie's invention, but even with having abstracted, or 

 caused to be abstracted, certain sheets of Mr. Leslie's work on Heat from 

 Gillet's printing office. 



It is needless to inform our readers that Count Rumford's thermo- 

 scope, and Mr. Leslie's differential thermometer are identically the same., 

 and that they both consist of a glass tube, bent in the shape of the letter 

 U, and having at each end a glass ball filled with air, while the legs 

 and horizontal branch contain a coloured fluid. 



We do not mean at present to decide the question between Count Rum- 

 ford and Professor Leslie, though the details which it involves furnish 

 much curious matter of scientific history. 



Sir Humphry Davy, in his Elements of Chemical Philosophy, publish- 

 ed in 1803, (pages 75, 76, and Plate I. Figs. 2, 3.) was the first to rectify 

 the history of the differential thermometer. This distinguished chemist 

 ascribes it to Van Helmont, who died in 1644, and he placed on the 

 same plate engravings of the instrument of Van Helmont, and of that 

 of Mr. Leslie. No sooner did this work of Sir Humphry's appear, than 

 it was attacked in a letter in the newspapers, bearing, we believe, Mr. 

 Leslie's name, which was ably answered by some friend of Sir Humphry 

 Davy's. 



Although no candid inquirer could doubt that Van Helmont anticipa- 

 ted the invention of the differential thermometer, yet there was one little 

 loop hole in his description of it through which a person of small di- 

 mensions might creep ; and in this way, from a slight flaw in the indict- 

 ment, the differential thermometer has been allowed to preserve a kind 

 of separate existence. 



Abandoning therefore the cause of Van Helmont, though a good one, we 

 are now prepared to jirove, that the differential thermometer was invented 

 by John Christopher Sturmius, Professor of Mathematics at Altdorff, who 

 published an accurate account of its construction and theory, illustrated by 

 drawings, in his Collegium Experimentale Curiosum, which was'published 

 at Nuremberg in 1676. Professor Sturmius calls it a thermometer and 

 also a thermoscope, the very name used by Count Rumford ; and he de- 

 scribes no fewer thanjive varieties of them, of which his own is the third. 

 The first thermoscope consists of a thermometer tube, with a ball at the 

 end of it containing air. This tube with the air ball uppermost, is placed 

 with its open end in a vessel of coloured liquor. The degree of heat, by the 



* Mr. Leslie's book on heat appeared in 1804. Count Rumford's paper was 

 read before the Royal Society on the 2d Ftbruary 1804. 

 VOL. II. NO. I. JAN. 1825. L 



