with Description of one of an Improved Form, 103 



then employed in the construction of the surveying instrument 

 called a Plane-table. 



According to Sir John Herschel, the cross-hair, which gives 

 so much accuracy to all astronomical as well as levelling in- 

 struments, was the invention of Gascoigne, a young English- 

 man, who used it in 1640. He was killed at the age of 23, at 

 the battle of Marston Moor. 



M. Le Bion* appears to have been the first to conjoin the 

 telescope of Huygens with the air-bubble of Dr Hooke ; and 

 this must have been subsequent to the year 1684, as such an 

 instrument is not shewn in De La Hire's edition of Picard's 

 Treatise on Levelling.t 



But it was not till Sisson's improvements that the level 

 could be considered as in any way an accurate or philosophic 

 instrument. All that were made previous to his time were 

 coarse instruments, adjusted by a ball and socket, and in other 

 respects resembling the common perambulatory survey-level, 

 which, from the nature of the construction, can be levelled in 

 only one direction, and cannot be reversed, or moved even in 

 the slightest degree, without requiring readjustment. Sisson 

 may, therefore, be considered as the inventor of the instru- 

 ment in common use. The main feature in his improvements 

 was the introduction of the four screws called the parallel 

 plate-screws (D, in the Diagram). I have been unable to 

 find out the date of Sisson's improvement ; and, indeed, the 

 only notice I can find of him is the following in Switzer's Sys- 

 tem of Water- works : *' The invention" (alluding to the in- 

 strument with parallel plate-screws) " as I take it (for I am 

 not as yet well acquainted with that gentleman), of William 

 Sisson, at the corner of Beaufort Buildings^ in the Strand." J 



Since the time of Sisson, the celebrated Ramsden intro- 

 duced a tangent-screw and clamp, for moving the instrument 

 with accuracy through small distances in an arimuthal direc- 

 tion. Messrs Troughton and Simms also made several im- 



* Traite de la Construction et des Principaux usages des Instrumens 

 de Mathematique. Par N. Le Bion^ Ingenieur du Roi pour les Instru- 

 mens de Math. Nouv. Edit. A La Haye, 1723. 



t Traite du Nivellement Par M. Picard, mis en lumiere par les soins 

 de M. De La Hire, 12mo. A Paris, 1684. 



% An Universal System of Water and Waterworks. By Stephen Swit- 

 zcr, 2 vols. 4to. Lond. 1734. 



