604 



SECTION 15. GEOGRAPHY. 



WEST GALLERY. KOOMS L. AND M 



I. SURVEYING INSTRUMENTS. 



COLLECTION OF INSTRUMENTS LENT BY THE ORD- 

 NANCE SURVEY. MAJOR-GENERAL CAMERON. 

 R.E., C.B., F.R.S., DIRECTOR-GENERAL. 



2943. Colby's Compensation Bars. 



The first operation in an extensive survey, viz., the measurement of a base 

 line, is beset with many difficulties, one of the greatest of which arises from 

 the fact that the lengths of the measuring rods, bars, or chains vary with the 

 temperature. This could be got over if one could ascertain at any moment 

 the precise ^emperature of the rods, bars, or chains, but it is not easy to be 

 assured on fnis point. Hence arose the idea of a compensation apparatus, 

 which was carried out by Major-General Colby, at the commencement of the 

 survey of Ireland, in the following manner. Two bars of different metals 

 and different rates of expansion are laid parallel and close to each other as 

 AB, CD 



A B 



I I I 



:C Di 



Pi IQ 



They are firmly connected at the centre, from or to which point they are 

 free to expand or contract. At a given temperature they are taken of the 

 same length, and in this state suppose lines AGP BDQ to be drawn through 

 their extremities, so that AP BQ make also the ratio of AP to CP and 

 of BQ to DQ, equal to the ratio of the expansion of the bar AB to that of 

 the bar CD. Now, if we suppose both bars to receive an equal increment of 

 temperature, and in this position lines to be drawn through their extremities, 

 these lines will pass through the points PQ. 



The compensation bar, then, consists of a bar AB of brass, united at its 

 middle to an equal bar CD of iron ; at each extremity of the bars is a metal 

 tongue, connected with them by pivots. These pieces are about six inches in 

 length, and on a silver pier at the extremity of each is a finely engraved dot. 

 The dots are 10 feet apart at all temperatures. 



When used in measurement of a line, the dots on two adjacent bars are 

 brought to the precise distance of six inches apart by means of a compensa- 

 tion microscope. The compensation microscope is formed by uniting two 

 ordinary microscopes by parallel bars of iron and brass, in such a manner 

 that their outer foci are " compensated," and measure six inches whatever 

 may be the temperature. The whole apparatus consists of six bars and 

 seven microscopes, of which two bars and three microscopes are now 

 exhibited. 



