36 Description of a new Levelling- Staff. 



venting a distinct view of the whole numbers on the staff, 

 and rendering it necessary in many instances to remove the 

 vane for the purpose of reading off, which subjects the 

 operation to much uncertainty, besides which, there is a 

 difficuhv in accurately bisecting the central stripe on the 

 vane, the wire in many cases subtending a larger angle 

 than the stripe itself. Many persons, with a view of ob- 

 viating these difficulties, are in the habit of observing the 

 top or bottom of the vane instead of the centre, and reading 

 off from the top or bottom accordingly : this, however, is 

 more objectionable and uncertain than that of hisectinc. the 

 central stripe; the wire in most levels subteudme; an angle 

 of several minutes, aiid therefore if brought in cont ct with 

 the top or bottom of the vane, the error will be equal to 

 the semi- diameter of the wire, which I have found in sht>rt 

 levels to be equal to an inch in a chain. Others have al- 

 ternately applied the top and bottom of the vane to the 

 under and upper part of the wire, and taken half the dif- 

 ference between the two readings as the true level point. 

 This, although an accurate, is a tedious meihod. 



The vane represented by the inclosed drawino (PI. T 1 ) will 

 obviate the necessity of having recourse to such modes of 

 practice in future. I have tried several forms, and find 

 this under all circumstances the best. 



The correct bisection of the cross is extremely easy, and 

 three inches of the scale bemg exposed, no error can arise 

 in reading off. 



The vane consists of a circular piece of brass 5 inches 

 diameter, and -'^th of an inch thick, having an opening of 

 3 inches in length, and ^th of an inch less in width than 

 the staff: before the opening is made, the cross is carefully 

 described, and the metal constituting it left entire. That 

 portion of the cross that falls on the plate itself, as well as 

 that which crosses the staff, is to be blacked like the hours 

 of a clock, and the remainder of the plate silvered, by which 

 means it will be visible at a quarter of a mile distance, and 

 the circle itself will easily be bisected at half a mile. 



There are two thin brass cheeks riveted to the back of 

 the circle for the purpose of clipping the staff, and for 

 moving the vane agreeably to the direction of the observer. 



The clamp usually fixed to the front staff should be flush 

 with the sides, in order that the vane may have its full 

 range without obstruction; which is not the case with many 

 of those now in use. 



I am, sir, your most obedient servant. 



No. 2, Tower Royal, Nov. 7, 1811. JoSEPH SXEEVENS. 



IV. On 



