SUPPLY OF SCIENTIFIC INSTRUMENTS. 217 



In the year 1879 Mr. Cushing brought out a new form of levelling 

 instrument which he called a "reversible" level. The objects were 

 to facilitate the adjustments, make the instrument more compact 

 and rigid, and render the surveyor independent of the aid of an 

 instrument-maker should anything go wrong when in the field. 

 Several hundreds are in use at the present time in India. 



The tide-predictor, constructed for the Indian Government by 

 Messrs. Lege and Co., is located in the observatory at Lambeth, 

 and tvith its aid tide-tables for upwards of 20 ports are annually 

 printed and forwarded to India towards the end of the year pre- 

 ceding the one for which they are prepared. The tides for several 

 ports are also predicted for the Colonial Office. This work is very 

 ably done by Mr. B. Eoberts of the Nautical Almanac Office.* 



During the year 1889 three new standard yard measures were 

 constructed in England by Messrs. Trough ton and Simms as 

 primary standards of length for the Governments of Bengal, Bombay, 

 and Madras, and were verified by the Standards Department of the 

 Board of Trade. It may be interesting to know that these important 

 standards do not absolutely agree in length, but they are probably 

 as near as human skill can make them. The standard temperature 

 of India having been fixed at 85° Fahrenheit, the certificates of 

 verification, which were forwarded to the respective Governments 

 with the standards, show their respective lengths at that temperature, 

 which are as follows : — 



Bombay - 36-00023406 inches. 



Calcutta - 36-00039041 „ 



Madras - 36-00007206 „ 



And the co-efficient of expansion in each case is the 0-0003744 of 

 an inch. 



The mints of Calcutta and Bombay were each supplied in 1889 

 with new sets of standard tola weights, all of which were verified 

 by the Standards Department of the Board of Trade, and the 

 tables of errors were supplied to the respective mints with the 

 weights in question. 



The Calcutta mint also received a large balance, constructed by 

 Napier, for weighing silver. It was a splendid piece of mechanism, 

 made to carry upwards of 380 lbs. avoirdupois in each pan and to 



* For a description of the tide predictor, see p. 199. 



