XI. — On the Trigonometrical Survey of Van Diemen's 

 Land. By Major Cotton, H. E. I. C. S. ; Deputy 

 Surveyor-General. [Bead lOth May, 1854.] 



On the 21st April, 1852, I made a report on the measure- 

 ments of the two Base Lines, and the result of the connecting 

 Triangulation, of which the following is a copy : — 



" Measurement of the two Base Lines, and result of 

 the connecting Triangulation. 



21st April, 1852. 



The Kods used for the measurement of the two Base lines 

 were of old Baltic fir, about fifteen feet in length and two 

 inches square : they were saturated with boiling oil, and var- . 

 nished, rolled in flannel and packed in sawdust, in coffers six 

 inches square, closed at the ends, but leaving the rods free to 

 contract and expand. The rods were supported centrically in 

 the coffers by means of blocks of wood ; the coffers aided by 

 these blocks serving to truss the rods. To the ends of the 

 rods were attached Brass Caps, rising to the level of the sur- 

 face of the coffer, and bearing on their upper surface the scales, 

 by means of which their lengths were determined. One cap 

 bore a zero mark only, and the other a vernier scale 19-20ths 

 of an inch divided into twenty parts. The only standard 

 then in the colony was a four-foot steel standard, divided into 

 inches and fortieths, and the vernier scales were made to 

 accord with these divisions, determining the measurements 

 to l-400th of an inch. Besides the vernier scale attached 

 to the cap of the rod, three similar scales were laid on the 

 surface of the coffer at intervals, and their several distances 



