3S On Ihe Bifferential Thermometer, 



metitum amblentis in canali, nisi unus gioborum sit aper- 

 tits, alter vero clausus." And further: "Sine apertura in 

 F liquor BC iion fuisset motus e Idco." 



It thus appears that Van HclnioiU's thermometer is de- 

 scribed by him as open, in the most explicit terms. But 

 to prevent the speedv dissipation oF the liquor, the aper- 

 ture or canal F is partlv closed by a stopper, as appears 

 from some other sentences in the description. This is not 

 very well rejjresented in the esigraving, which is only a 

 rude outline, and advantage is taken of this by A. B. n- say 

 that " there is no aperture delineated, and it miuht be con- 

 ceived to be hermetically sealed." It is true that, iVum a 

 view of the figure alone, it might not be evident what is 

 meant to be represented at F. But a rel'erence to the de- 

 scription which is immediately adjoining, the fii^ure itself 

 being in the text, points out by tlie most cxjilicii statement 

 that it is " a canal open at its exlremitv." This defence 

 then by A. B. is merely a subterfuge. And it does not after 

 all justify Sir Humphry's representation ; for, in the figure 

 he gives, there is no part corresponding to the canal F, and 

 the instrument is represented hermetically sealed, without 

 the possibility of inferring that it might be open. Why 

 was this alteration made ? The effect of it is to render the 

 thermometer of Van Helmont essentially the same with 

 Mr. Leslie's. But it would be hard to say that this had 

 been the intention, and we must therefore suppose that 

 some other adequate cause can be assigned. 



The remark by your correspondent, that Mr. Leslie has 

 introduced a cork, is rather frivolous ; for whether a cork or 

 a stopper is put into the canal F is of no importance; the 

 important point which Van Helmont is so careful to slate 

 is, that there shall be an opening there. Mr. Leslie savs 

 nothing of a cork ; and the error ui the delineation of the 

 instrument, alluded to by A. B. is so trivial that it does not 

 deserve notice. 



With regard to the form which Van Helmont gives to 

 the instrument, and from which he calls it " suum or- 

 ganum," it is merely turning up the common air ther- 

 mometer af the h(,ttom, and giving it a ball, so as to iorm 

 a bason, in wtiich the liquor when depressed in the stem 

 might be contained ; a variation introducing no essential 

 difference into the instrument: and it may be remarked, 

 that nothing was more common than to vary the form of 

 the air (hermometer, as mav be seen by a reference to the 

 works of that period. Fig. 3. for example is the form of 

 one given by Sanctorio in his " Commentary on Avicenna.'* 



The 



