On the Differential Thermometer. 3 1 



excess of the total arc is extremelv smill, as it does not 

 exceed \",39,, so as to make but (Ive or six toises difttreiice 

 ill the length of a decree observed on the meridian, and 

 corresponding to the mean latitude of the arc examined. 



[I'll beconlinued.] 



VII. On the Differential Thermometer. 

 To Mr. TiUoch. 



Sir, X he simplest mode of settling the point in dispute 

 between Professor Leslie and Sir H. Davy, with regard to 

 the invention of the differential thermoinetcr, is to give 

 exact representations of Mr. Leslie's instrument, of Van 

 Helmont's, and of the figures Sir Humphry has given of 

 them. 



Fig. 1. (Plate II.) is the differential thermometer of 

 Mr. Leslie copied from the figure he gives of it in his 

 work on Heat. Fig. 2. is the instrument of Van Helmont 

 copied from the edition of his works 1648, and of which 

 the figure in the edition 1652 is the same. Fig. 3. is the 

 representation Sir H. gives of Van Helmont's instrument; 

 and fig. 4. is that which he gives of Mr. Leslie's. No one 

 I suppose can look at these figures without perceiving that 

 they are not correct representations either of the one or of 

 the other, but that both are essentially altered. The -simple 

 question, and which one would imagine might be easily 

 answered, is, Why were these alterations made? 



Instead of explaining this, a correspondent A. B. in your 

 journal for November has said, as affording a ground of 

 justification of Sir Humphry, that in the figure of Van 

 Helmont " there is no aperture delineated, and it might 

 be conceived hermetically sealed." He adds, that in the 

 representation Mr. Leslie gave of this instrument in your 

 journal, he introduced, a cork, and his figure differs from 

 those in the two editions of Van Helmont, which are found 

 in the library of the College of Edinburgh, and which are 

 perlectly alike. 



To judge of the validity of this defence, it is only neces- 

 sary to read Van Helmont's description of the instrument. 

 " A et D sunt du« spherse repKtos acre, A aulem et su- 

 perior est exterius undcquaque clauKa, D vero est globus 

 inferior apertus in C\u^ canalis V. Sunt autem A ct D ex 

 nnico vitro conncxa per canalem liCE." He afterwards 

 adds, " Liquor BC non potest se movcrc per tempera- 

 ment um 



