On the Differential Thermometer. 329 



quantities alone ? The attraction ( F ) of the earth, on a 

 particle at any distance {y) from its centre, is eniireiy de- 

 terminedly y alone: but there can \^ii no equation wWh- 

 out the assistance of other quantities. 



Let 'P and T be any fixed or given values of F and y, 



we have F : <? : : ^ '■ ^^^ whence F = ^ X ^ x -. . 

 Here then is an equation ; but between F and y alone, we 

 could only have an analogy ; viz. F a -5-. The case is ex- 

 actly similar with respect to the side of a triangle, and the 

 three angles. Thout^h there can be no equation betvxeeii 

 c, A, B,°C alone; ytt, if x, a,/3, 7 be fixed values of those 

 quantities, there is no reason why we might not have 

 C : f (A, B, C) : : X : ^ («, /3, y) ; whence 



c — — .'^ X a'AjBjC); and even if we should grant, 



(p ! a- /3. y i 

 to pkase tlie Edinburgh Reviewer, that A,B, C are of the 

 nature of numbers, it will not hinder each side of this 

 equation from Ijeing of one dimension. But [confess I 

 have scarcely paiience to speak seriously on a subject which 

 every one but the Reviewer' sneers at : and he, - fancy, will 

 not shortly be induced to discuss this topic a third time. 



LVI. On the Differential Thermometer. 



To Mr. Tilloch. 



Sir, Mr. Leslie has stated that the curious instrument 

 described by Van Helmout is the same as the air thermo- 

 meter of Drehbel or Sanctorio. If this was the case, as 

 Van Hthiiont was of the sanip age as those men, and as he 

 has not quoted ihem^ but calls the inslrument his own, 

 « suum organuni," it might be still a question, whether 

 he was not an inventor; but whoever will compire Dreb- 

 Lel's Thermometer as copied in Shaw's Translation of 

 JJocrhaave's Chemistry, and Sanctorio's as engraved in hi& 

 Wdiis, fol. Wueliis, i>ag. 22, with Van Hclmont's. will 

 find them whoHv unlike. Sanctorio's (fig. 1. Fl. VIH.) 

 and that attributed to Drebbel (fig. 2) cou'^isted ol a single 

 gl )be and an open tube, and the end of the tube was ii.tro- 

 duc»d into a vessel containing colouittl liquor. In the 

 figure attached to the first editi"n of Van i ielmont, pub- 

 lished after his death in ;648, of which lig. 3. is a copy, 



• Anticyram ratio illi dcstinat owifw. 



there 



