102 Mr T. Stevenson on Levelling Instruments, 



fixed to his works,* in the following terms : — " Ibi" (Lutetise) 

 " vixit ab anno 1666 ad annum 1681. Durante hoc tempore 

 pulcherrima subtilissimaque multa in mathematieis detexit 

 variaque ex iis operibus conscripsit quse nunc in unum corpus 

 collecta quid in variis Matheseos partibus prsestiterit sub oculis 

 ponunt. Praeter ipsius jam memorata inventa prseclara inter 

 alia duo insigni usu eminent. Libellam telescopio munitam 

 ita construxit ut ipsi pras ceteris fides haberi possit," &c. 



The honour of having first applied the air-bubble to the de- 

 termination of horizontality seems to be due to that universal 

 genius Dr Hooke. From all that I can gather, it appears that 

 his invention must have been made subsequent to 25th March 

 1674, and prior to the year 1675, as, in his '* Attempt to prove 

 the Motion of the Earth by Observations,*' of date 25th March 

 1674, he describes a new method of stilling the plummethy im- 

 mersion in water. While in his animad versions, f published also 

 in 1674, after fully describing his invention of the air-bubble 

 confined in a tube, he speaks of its peculiar advantages, and 

 great delicacy of movement, and remarks, — " This can hardly 

 be performed by the ordinary way of plummets, without hang- 

 ing from a vast height, which is not practically to be per- 

 formed without almost infinite trouble, expense, and diffi- 

 culty," &c. 



Hutton, in his Mathematical Dictionary, remarks, that the 

 application of the air-bubble to the level " is said to be due 

 to M. Thevenot ;" but with what justice I cannot say, having 

 been unable to meet with any reference to this instrument in 

 the writings of that author. Thevenot was born in 1621, and 

 he died in 1692. 



I have been unable to discover who was the inventor of the 

 circular level, which I imagined had been of recent date ; but 

 Switzer, at page 91 of his Treatise on Water-works, which 

 was published in 1734, remarks, that the circular level was 



* Christ. Hugenii Op. Var. Lugd. Batav. 1724. 



t Animadversions on the first part of the Machina cselestis of the Hon., 

 learned, and deservedly famous Astron. Johannes Hevelius, Consul of 

 Dantzick, together with an explication of some instruments made by 

 Rob. Hooke, Prof. Geom. in Gresh. Coll., and F.R.S. Lond. 1674, p. 61, 

 et seq. 



