TRIGONOMETRICAL SURVEY OF INDIA. 



203 



and iron bars do not cool equably, if their natural surfaces be exposed to 

 the air. Hence it has been necessary to apply coatings of varnish so as 

 to give surfaces which shall make them radiate alike, and as this varnish 

 must be only applied partially, the quantity, which is just sufficient for 

 the purpose, and no more, must be patiently ascertained by a series of 

 repeated trials. 



Strictly speaking, the distances from the neutral points on the tongue 

 to the centres of the two points ought in every position to bear the same 

 proportion to each other as the increments of the brass and iron bars do, 

 but a very little consideration will suffice to shew that the increments of 

 the tongue itself will not be sufficient to answer that condition ; the error 

 must however be one of the second order, and has hitherto been considered 

 as rejectaneous, which it most probably is. 



The compound bar thus formed, is placed upon two brass rollers 

 which revolve each upon an axis fixed into a deal-box, whose length 

 is sufficient to enclose the whole metallic apparatus, except the tongue 

 which protrudes at one side, and is protected by a cylindrical cover 

 of brass tube, part of which turns round and brings a circular aper- 

 ture over the dot marking the neutral point. The box is of deal 

 and painted white on the outside, it is lined with white cloth, and 

 there is an inner box covered on both surfaces with white serge, so 

 that a stratum of air may pass between the outer box and its interior 

 lining. 



At each end of this deal box there is a common brass sight which 

 serves to lay the bar approximately; there is also a brass plate with 

 screw holes, by means of which aic fixed to the box the arms intended to 

 support the microscopes. 



