352 SURVEY OF THE COAST 



The measuring of the angles can of course be varied, and the 

 cords of the two vertical angles can always be measured, and 

 in many cases the supplements. It will be more proper to 

 let every angle be composed of two, given by tiic cords of 

 two angles, goi g oft' from a fixed position of the lower rule, 

 than to place the lower rule together wiih the upper upon a 

 point, with their points placed upon each other. 



Instruments such as I have now described at length, I had 

 executed in i79Ji, by the exact artist Develey in Lausanne 

 in Switzerland, for the Surveyors of the Comissaiiat General 

 of Berne, as we were not satisfied with the usual alhidades 

 with lights. These instruments have, m all respects, an- 

 swered very well, and liave not deteriorated by long use. 



The plane table itself ought to be about tliirty inches by 

 twenty-four in size, as light as it can be made consistently 

 with solidity, it may, on that account, be proper to have it 

 pannelled. 1 have always found that old pine board, which 

 had served long as doors or house furniture, ^"c. was the 

 best material for it. The size above mentioned allows pa- 

 pers of such dimension as will be found advantageous, while 

 a small table will introduce inaccuracy, by the necessity of 

 changing often, and adjusting many papers. For the same 

 reasons plane tables, with frames to stretch the paper upon, 

 are to be rejected. The paper must be allowed to be of 

 great length ; and a breadth of three feet and a half may well 

 be placed on tiie table, of thirty inches in one direction. It 

 is good to have the edges rounded oft", so tiiat when the pa- 

 per is wound round it, the part not used may be rolled up 

 under the table, and kept from Iblds or bends. 



The paper is stretched and held upon this table by brass, 

 or (which is still l^etter) steel springs of sufficient sfrengtli, 

 and of the shape and about the size seen in Plate IX. fig. (5 

 ^ 7. These springs sliding over the edges of the tal)le. and 

 holding in front, admit freedom to the paper arcnind the ta- 

 ble, l)y the greater width of the round sprinii part behind. 



The motion work of the plane table is exactly similar to 

 that of the large needles, and may be seen in Plate IX. fig. 3. 



