On the Use of the Aneroid Barometer as an Orometer. 305 



led the author, as Chief Engmeer m charge of the Expedition, to 

 provide (among other instruments) some aneroids, as a means of 

 sa'Ndng time in ascertaining the levels of the mountain passes of that 

 Island. The aneroids oifered by makers did not appear sufficiently 

 graduated to admit of minute observation, and at the author's sug- 

 gestion INIessrs. Elliott furnished a more complete vernier, which, 

 however, was shown to be susceptible of material improvement. 



With these comparatively imperfect instruments, it was shown 

 that an elevation of 950 feet may be taken to correspond with the 

 fall through the first inch of the aneroid ; that about 970 feet more 

 corresponds with the fall through the second inch, and about 1000 

 feet corresponds with the fall through the third inch. These altitudes 

 having been checked by levels taken with the ordinary surveyor's 

 spirit-level, it was shown that this experience corresponds with the 

 Tables published by ]\Ir. Belville, within 1 per cent. 



The thermometer, which is usually attached to the aneroid, is not 

 a necessary adjunct, but is frequently useful, and always interesting. 

 The compensations introduced to provide against variations of tempe- 

 rature, as affecting the results given by the instrument, were shown 

 to be effectual without the aid of the thermometer. 



The difficulties to be contended with in taking accurate observa- 

 tions were shown to be local variation, diurnal variation, and some 

 irregularity in the action of the mechanical parts of the instrument 

 itself. These difficulties were examined seriatim, and modes of ap- 

 proaching to their corrections were explained. The modes of com- 

 pensation for variations of temperature affecting the instrument were 

 shown as at present practised by the makers : the diaphragm-box 

 being compensated by means of the introduction of a small portion 

 of aeriform fluid, instead of being allowed to act with a perfect va- 

 cuum, and the metallic connexions between the diaphragm-box and 

 the index being compensated by compound arms or connexions of 

 steel and brass so adjusted as to neutralize mutually the respective 

 contraction or expansion of each at variations extending to 100 de- 

 grees of temperature. 



The mode now practised by makers of graduating the aneroid 

 (when thus compensated) by com])arison v.'ith a standard mercurial 

 barometer, was stated, and it was suggested that improvement on this 

 practice might be made by reference to standard elevations running 

 up to 2000 feet at least in Great Britain. Practical examples were 

 given of the use of the instrument in Ceylon, shovving the variations 

 of the aneroid (when properly checked) to lie between 1 foot and C 

 feet, as compared with the surveyor's s]jirit-level : other examples 

 were given of practice on the Great Western, South-Eastern, and 

 North Kent Railways, varying from the true levels from G inches 

 to 6 feet, over distances of between oOO and 400 miles. 



March 4. — The Lord Wrottesley, President, in the Chair, 

 " On the Stratifications and Dark Bands in Electrical Discharges 



as observed in Torricellian Vacuums." [The Bakcrian Lecture.] 



By John P. Gassiot, Esq., V.P.U.S. 



The author refers to tlie stratified ajjpearancc of the electrical dis- 



rhd. May. S. 4. Vol. IG. No. 107. Oct. 1858. X 



