TRANSACTIONS OF THE SECTIONS. 37 



dently favourable to uniformity of temperature in the rod and mercury, 

 &c. &c. 



All strain or warp of the spring, in the final suspension of the pendu- 

 lum, can be avoided. The pendulum being first suspended freely, is 

 left until, by the cessation of its action, it arrives at its own line of rest 

 in every direction, particularly in that which passes through the plane of 

 the spring. The fixing-piece is then brought to it and the whole per- 

 manently attached togethei". 



The line of the flexure of the spring can be determined and preserved. 

 Usually, the exact position of this line is, within certain limits, left to 

 accident, and is, from several causes, continually changing its position ; 

 consequently, the pendulum is simultaneously varying in length. Er- 

 rors in rate, often attributed to other causes, are the necessary conse- 

 quence. 



Notice of a cheap and portable Barometrical Instrument proposed for 

 the use of Travellers in Mountainous Districts. By Sir John 

 RoBisoN, Sec. R.S.E., ^c. 



The instrument is a glass tube about 0*25 of an inch in diameter, 

 and about 14 inches long, with a small bulb like that of a thermometer 

 blown on the upper end. The stem of the tube has been graduated by 

 divisions made experimentally by the instrument-maker in the follow- 

 ing manner. On a day when the barometer stood at 30 inches, and 

 the temperature of the air was 62°, it was placed in the receiver of an 

 air-pump, and when the rai-efaction allowed the barometer of the 

 pump to fall to 29 inches, the instrument v/as lowered until the open 

 end of the tube became immersed in a cup of water, over which it had 

 been suspended by a thread and wire passing through a stuffing-box. 

 On allowing the atmosphere to enter the receiver, the water was jDressed 

 up the tube until the density of the air corresponding to 30 inches 

 was restored, and the height of the fluid was carefully marked. The 

 instrument was a second time suspended in the air-pump receiver, and 

 the exhaustion was repeated until the barometer gauge indicated 28 

 inches, the immersion in the cup having been made as formerly ; the 

 air rushed in, and the graduation of the tube corresponding to 28 

 inches was accomplished. By continuing this process, the graduation 

 of the stem was carried on as far as was thought requisite, (the inter- 

 mediate divisions having been made in a similar manner,) when the in- 

 strument became ready for use. 



It is obvious, that in this manner a traveller arriving at a station in 

 the midst of mountains, and having with him a number of such tubes, 

 would only require to send messengers to their summits with one or 

 more of these tubes and a tin case containing water, for the purpose of 

 giving him the means of determining their heights with considerable 

 accuracy. Each messenger, carrying with him the empty glass tubes, 

 is to be instructed to insert their open ends in his flask of water when 

 he shall liave i-eached the summit, and to bring them down again. 



