1869.J The Lambeth Observatory. 349 



the existing appliances this comparison is carried, not without 

 difficulty, up to 92 degrees of Fahrenheit. It is deemed most 

 desirable, where service in India is contemplated, nevertheless to 

 extend the examination of intermediate points up to 120 degrees. 

 But this is a very delicate and difficult task. Whenever there is 

 more than 30 degrees between the natural temperature of the air 

 and the temperature of the water in which the instruments are 

 immersed, the cooling of the water goes on so rapidly and so 

 irregularly that instruments immersed in the liquid are really 

 differently affected in different places. A remedy for this imper- 

 fection is under consideration, and it is intended that means shall 

 be provided, as opportunity allows, for testing large numbers of 

 thermometers at a time in which the uniform diffusion of tempera- 

 tures quite up to 120 degrees is artificially provided for in a vessel 

 surrounded by a large body of heated and constantly moving water, 

 the heat of which is to be retained by a jacket, or coating, of very 

 slowly conducting material. The freezing points of four standard 

 thermometers, which, after all, are the sole reUable and unalterable 

 elements, are examined once a-year to guard against error from the 

 contraction of the bulbs and other slow changes of constituent 

 material. 



The instrument which occupies the place of scrutiny in this 

 testing observatory at the present time is one of a very magnificent 

 set that has been for some time preparing for use at the central 

 stations of the great Trigonometrical Survey of India. It is a 

 large theodolite (one of a pair), with a three feet vertical, and a two 

 feet horizontal, circle. Its companions for this service, of which 

 specimens stand in reserve in other parts of the observatory, are a 

 pair of transit instruments, a pair of zenith sectors, and a pair of 

 chronographs. In many particulars these noble instruments are 

 entirely unique, and deserve a much more precise and particular 

 description than can here be given, in consequence of hmitation of 

 space. 



It may, however, be nevertheless briefly explained in regard 

 to them, that each theodolite, which is designed mainly for deter- 

 mining the angles of large triangles through the instrumentality of 

 heliotrope flashes given by mirrors from twelve to forty miles away, 

 has a frame chiefly composed of aluminium bronze, which is three 

 times as rigid as a similar structure of gun-metal, and therefore 

 confers twice the rigidity, in two-thirds of the weight that gun- 

 metal can furnish. The large circle bearing the graduations for 

 fine work is protected by an outer guard-ring which takes all the 

 strain, and serves also to equahze temperature. The tangent screw 

 is made, by the application of a peculiar spring, to exert an 

 invariable force. The adjusting levels are of great dehcacy, and 

 the telescope is furnished with two eye-pieces, one with a horizontal 



2 B 2 



