1851.] The adaptation of Aneroid for surveying in India, 321 



ing to thousands will indicate ten feet. The Mountain Sympiesometer 

 scarcely seems to have been made use of at all amongst us ; wherefore 

 I know not : the instrument is quite as portable as the Aneroid : it is 

 much less susceptible of injury than the Mountain Thermometer, but 

 is apt on being long used in this country to change its rating. Both 

 the Aneroid and Sympiesometer in their best forms require to be 

 verified by frequent reference to a standard barometer. The Mountain 

 Thermometer has this advantage over both that once rated it runs 

 little risk of going sensibly wrong. I say sensibly, for by a late paper 

 of Mr. John Adie's it appears that even Thermometers in the course of 

 time alter their indications.* The Mountain Thermometer, portable as 

 it is, is far from being exempt from accidents, and besides being apt to 

 be broken in carrying about or in heedless handling while being boiled, 

 the air is liable to get entangled with the mercury, an accident often 

 occurring to such an extent as to occasion the risk or destruction of the 

 instrument.f 



* Mr. John Adie of Edinburgh has published a very elaborate article in the 

 Edinburgh Philosophical Journal of January, 1850, on the change which takes 

 place in the starting points of Thermometers, often amounting to no less than nine- 

 tenths of a degree in a few months ; this is equal to 450 feet in elevation, suppos- 

 ing the thermometer to be cut to tenths, there being no means of detecting or re- 

 medying the error. I do not think any Aneroid or Sympiesometer likely under any 

 circumstances to go wrong to the extent of half an inch, nearly the equivalent of 

 this, if they have been tolerably taken care of from the time of their last rating by 

 the standard Barometer or reference to some point of known elevation. 



f The following description is given by Mr. Adie, of the Mountain Thermometer 

 as supplied by him to the Bombay Geographical Society. " The Thermometers 

 for the determination of altitudes by the boiling point of water are constructed as 

 follows : A piece of tube is selected of perfectly equal calibre throughout its 

 length ; the section of the bore is round and fine, for the purpose of giving long 

 degrees without having a very large bulb, which renders the carriage of such Ther- 

 mometers, very dangerous for breakage ; the bulb is made of glass cylinder tube, 

 which can be made more equal and stronger than a round bulb : and the proper 

 size having been determined for each tube, the scales are determined by the follow- 

 ing process : each tube with its finished bulb is weighed by a fine balance to 1.100th 

 of a grain : they are then fitted with pure dry mercury and regulated so that 62° 

 shall have the same position as 212° is to have when the Thermometer is finished. 



Temporary scales, divided into inch and decimal parts, are then fixed to each 

 tube, and the point 32° obtained from melting ice, and 62° from a fine standard 

 Thermometer, and carefully read off on these temporary scales. This gives the 



