On the Trigonometrical Survey, dec. 83 



and the total length of the rod measured by the four-foot 

 standard. Each rod when in use was supported on two 

 tressels fitted with screw lifts, affording the means of raising 

 and depressing them into their position. 



The rods were made in damp weather, and were used 

 during damp weather in the first measurements of the Base 

 at Ealph's Bay in 1849. They were measured as soon as 

 completed, and from time to time during the operation, and 

 the variation in length was scarcely appreciable. Since 

 this measurement was made, a ten-foot steel bar has been 

 received from England, by which the four-foot standard has 

 been tested and found to be very true, so that the mea- 

 surement requires no reduction on that account, and any 

 difference found to exist between it and subsequent re- 

 measurements must be imputed to the apparatus used and 

 to carefulness of operation. The Base was marked out with 

 pickets, placed in line by means of a transit instrument, and 

 divided into convenient gradients or hypothenuses, perma- 

 nent marks, over which the altitude and azimuth instru- 

 ments could be placed, being established at each extremity. 



The rods were then placed on their stands, and arranged 

 into the vertical plane of the Base line, and at the inchnation 

 of the first hypothenuse, the first rod within a few inches of 

 the terminus, and two others in succession at similar intervals, 

 the zero mark on one rod being antagonist to the vernier scale 

 of the next. The intervals betwen the rods were measured by 

 means of a small scale engraved for the purpose, which with 

 the vernier indicated inches, twentieths, and four-hundredths 

 of inches. The inclination of each hypothenuse was ascer- 

 tained by levelling, the rise and fall being entered in the 

 field book in feet and decimals. The horizontal value of 

 each hypothenuse was then obtained, and the requisite 

 reductions to the level of the sea computed. 



