34 On llie Differential Thermometer. 



ment without displaying its advantages, and pointing out 

 some use to which it was to be apphtd. He docs no such 

 thing. He is merely led, in the course of his illustration of 

 his own instrument, to introduce an observation; in which 

 observation, we, now that we are acquainted with the dif- 

 ferential thermometer, perceive its principle. But he 

 thought of no such instrument ; nor did the state of know- 

 ledge at that time furnish any object of experiment to 

 which it could have been applied. Mr. Leslie's invention 

 originated from difierent views ; it arose from the necessity 

 of obviating the inconveniences of tbe common air ther- 

 mometer to admit of its being applied to a particular sub- 

 ject of experiment; and it is injustice to him to represent 

 the observation of Van Helniont as an anticipation of his 

 invention. 



If Sir H. Davy had remarked, that in the course of a dis- 

 pute on a form of the air thermometer Vr^n Helmont had 

 incidentally stated the principle of the differential thermo- 

 ineter, Mr. Leslie could not have comf)lained. And had 

 Sir H. been eatrer to make this known to the world, he might 

 without impropriety have quoted the passage as it stood*. 

 But why say that Van Helmont (igured an instrument 

 very similar to the differential thermometer, when he 

 figured no such instrument ? Whv alter the figure of the 

 instrument which Van Helmont does give? VVhy alter it 

 in such a manner as to bring it to resemble Mr. Leslie's? 

 And why alter Mr. Leslie's so as to bring it, on the other 

 hand, to resemble more nearly the one said to be Van Hel- 

 mont's ? The^e are the true points of discussion ; it is 

 needless to wander from them ; and if Sir Humphry, or his 

 friends, give satisfactory answers with regard to them, he 

 will then stand exculpated from the charges which al pre- 

 sent lie against him. 



Some of yo'ir readers may think that more attention 

 has been bestowed on this subject than it deserves. Others 

 probably will be of a difflrent opinion. The praise due to 

 invcn'ion is often its sole reward; and any unfair or in- 

 vidious attempt to lessen it, ought both in justice to the 

 individual aggrieved, and from regard to the interests of 

 science, to meet with due reprobation Where there is any 

 appearance of this, therefore, it ought not to pass unno- 



* Such is the apparent eagerness of Sir Humphry on thi- point, thst he 

 introduces tv. Ice, first in tlie iiitroiiuction, and afterwanis in the body of 

 his work, the sta'en.enf, that Van Hehnont had given a c'^etch of an instru- 

 tucat similar to tiie diirereutial ihetmometer. 



ticed, 



