with Description of one of an Improved Form. 105 



Having thus endeavoured to describe the successive changes 

 which the level has undergone, I shall now proceed to notice 

 the nature of the present improvements. 



The first of these is the substitution of a circular, or, to 

 speak more correctly, a spherical level (G), sluggish in its 

 motions, instead of the small cross-level, which was introduced 

 by Mr Gravatt. The advantage of the circular level over the 

 common form, is its peculiarity in at once shewing the devia- 

 tion of the instrument from horizontality in both directions, 

 instead of only one. 



Before describing the next improvement, it may be proper 

 to state, that the clumsiness of the common level consists in 

 its being at all dependent on the setting of the legs. This 

 arises from the circumstance of the ball-and-socket motion (K) 

 being controlled in its action by the parallel plate-screws of 

 Sisson (D), the consequence of which is, that, in using the 

 common level, care must be taken to set the instrument very 

 nearly level by the eye^ so as to be within the range of the 

 parallel plate- screws (D), otherwise it is impossible to adjust 

 the instrument. And although to the practical man, the trou- 

 ble attending this may be comparatively small, still he will 

 admit that it is one of the most irksome parts of the whole 

 operation of levelling ; to say nothing of the time that is lost 

 in adjusting the instrument afterwards with the parallel plate- 

 screws. What appeared to be wanting was a motion for the 

 preliminary, or rough-setting^ intermediate in nicety between 

 those of the parallel plate-screws and of the legs. In order 

 to gain this end, a ball-and-socket motion (F), having a clamp 

 (N), is introduced in addition to the ball-and-socket (K), whose 

 action is limited by Sissou'^s parallel plate-screws (D) ; so that 

 my improved level has two ball-and-socket movements. 



With the instrument thus improved, the observer is made 

 quite independent of the level of the ground where he sets the 

 legs of his instrument, and may place them without regard to 

 the inclination of the telescope to the horizon. Looking first 

 to the circular level (G), and releasing the clamp (N) of the 

 ball and socket (F), he, with one hand, moves the head of the 

 instrument till the bubble is in tfie centre of the circle, an 



