0« the Differential Thernyometer. 33 



The syphon barometer was one of the first variations of the 

 coniniop Dai-umeter, it heino; more convenient to have the 

 bason /oriinng part of the tube than detached. Van Hel- 

 mont s variation -vnh regard to ihe air thermometer is just 

 the same; us under ball is open, and it could not be other- 

 wise .ndeed. Irom the purpose to which he describes it as 

 applicao.o, that o< showing the temperature of the surround- 

 ing mcamm. Hence he loads Heer with reproaches for his 

 stupid, -y in supposing it to be closed. If Heer however 

 IS tc be ra ik<d as an idio!, as Van Helmont says, (Heer 

 auteni apiid idiotas ostendebat,) for this miatake,' it would 

 De still wor^^- ,( the same mistake were now made after the 

 very au,p|.-, LXj)',.sure of it by Van Helmont himself. It is 

 indeed nnpossible that it could be made bv anv one who 

 fiad read any part of the passage in which 'the instrument 

 IS described. It would be worse than a mis^lake. The only 

 possible defence for Sir Humphry is, that he had never read 

 tne description ; and the same defence, whatever stretch of 

 candour it may require, must be extended to A, B. 



To the remark by your correspondent, that Mr Leslie 

 has not (pioted the sentence from Van Helmont in winch 

 the principle of the differential thermometer is described 

 and to the msiiiuafion conveyed in the additional observa- 

 tion that he "shall not pretend to determine whether Mr 

 i^eslie has not read this passage, or whether he has read it 



r^n l'?^l T^'^'• P"""!'"' ^" 1"'^'" ■''" 't '^ sufficient to 

 reply, that Mr. Leslie, m complaining of the injustice that 

 tiad been done him, confined himself to the subiect of his 

 complaint, which he stated as briefly as possible. He was 

 not called 011 to quote this passage more than some others • 

 Dut that he did not seek to conceal it, as A. B. would insi 

 nuate, is evident from this, that he admits all that can be 

 in erred from it,_that the statement of Val Belmont "in- 

 cidentally involved the principle of the differential thermo- 

 meter. f his is the precise statement of the fact, and it 

 does not invalidate Mr.Leslie's claim to the invention A 

 train of rcasoamg involving the principle of an instrument 

 or staling incidenlally ihat principle, is very difTerent fioiti 

 the aclua construction and applicution to use of ihc instru- 

 ment itself. Had the thermometer of Van Helmont been 

 such as Sir H. Davy has figured it; had it been true that 

 both Its balls are closed, and that it "exhibits the action 

 of heated upon cold air." it would have been an anticipa- 

 tion of the diflcrential thermometer; and Van Helmont 

 we may be ceriaiu, would not have given such an in<=trn- 

 Vol.41. No. 17 7.^««. Ibl3. C iiKnt 



