TRAXSACTIONS OP THE SECTIONS. 595 



tion on its stand, and the sights e f directed to the top of a, 

 post of equal height with the instrumentj at the other extremity 

 of the slope. 



The instrument, when placed in a horizontal position, serves 

 as a plane table ; in this case the divisions on I K, and the 

 rectangular coordinates, offer peculiar facilities for the execution 

 of a rough survey. The distance between two stations being 

 found on the scale I K, the readings on the rectangular lines 

 will at once give the easting and northing of the undetermined 

 station ; these can at once be transferred to a skeleton, prepared 

 by tracing squares on a piece of paper, and each successive 

 step of the survey is thus completely indicated on the map. 

 The bearings of the different lines must, of course, be noted, in 

 order that, by setting back upon them, the observations may 

 be rendered independent of any changes in the magnetic direc- 

 tion. 



On aChronometer with a Glass Balance-spring. By E. J. Dent. 



Mr. Dent presented an account of the rate of this instru- 

 ment, kept at the Royal Observatory, Greenwich, since the last 

 meeting of the Association. He shewed a chronometer in 

 motion, with a pure palladium balance-spring ; and produced 

 a table of the variations of gold, steel, palladium, and glass, 

 from 32° to 100° Fahr. ;and another table of the quantities 

 respectively due to direct expansion, and to loss of elasticity, 

 in steel and palladium. 



On the Polyzonal Lens. By Mr. Gordon. 



Mr. Alexander Gordon exhibited Moritz's modification of 

 Fresnel's polyzonal lens, which (with a common Argand flame) 

 is proposed as an ceconomical light for ports and harbours, and 

 to be adopted (when a more intense flame is used) for coast 

 lighthouses, in situations where the use of parabolic reflec- 

 tors is not absolutely necessary. 



On an Instrument for taking up Water at great depths. By 

 Mr. Rennie. 



Mr. G. Rennie described the principle of construction, and 

 the practical method of employing this instrument, which has 

 been tried by him at the estuary of the Tamar, near Plymouth, 

 and found to succeed completely. 



2 q2 



