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permitted, 'oy liavingthe needle on which it is fufpended as hard 

 and finely pointed as the weight to be hung on it will allow, 

 and the needle-cap properly made with a fine apex and polifhed ; 

 for befide the error of adjuftment, which may while it is ufing be 

 deteded, and either remedied, or the obfervation made truly, even 

 when that error remains, (by taking the mean of the greateft and 

 leafl :'altitudes the level affords in a femi-revolution of the mirror 

 round its axis, as has been obferved before), it is liable to no 

 other error but from a pendulous motion of the weight or a tre- 

 mulous motion of the inftrument itfelf from its being agitated by 

 the wind ; and if it is affeded by either of thefe, it will be vifible 

 by the vibration or tremor of the image in the level compared 

 with that in the quadrant which remains motionlefs. And I have 

 found that it refifts the adion of the wind fo well as to admit 

 obfervations to be made in almoft any weather in which a perfon 

 would chufe to take them in the open air. 



But I have obferved its fteadinefs to depend on the following 

 circumftances : 



ift. That the whole inftrument be as light as is confiftent 

 with the neceffary ftrength, efpccially the parts remote from the 

 point of its fufpenfion, as the edges of the mirror and the lower 

 part of the frame and ftem ; on which account the mirror Ihould 

 be a thin plate of glafs, and cannot be more than three inches 



in 



