143 



the concentration of radiant heat, as in the beautiful experiments 

 described by Dr. Tyndall in the commencement of his paper, will 

 rise very considerably in temperature, while the vesicles enlarge under 

 the continued influence of the heat received by radiation through the 

 cooler enveloping ice and through the fluid medium (air and a watery 

 film, or water) touching if all round, which is necessarily at Cent, 

 where it touches the solid. 



I find I have not time to execute my intention of sending you to- 

 day a physical explanation of the blue veins of glaciers which occurred 

 to me last May, but I hope to be able to send it in a short time. 



WILLIAM THOMSON. 



Jan. 21, 1858. 



II. "On the Practical Use of the Aneroid Barometer as an 

 Orometer." By Captain W. S. MOORSOM, Member of the 

 Institution of Civil Engineers. Communicated by P. W. 

 BARLOW, Esq. Received January 28, 1858. 



A Government Commission to Ceylon in the beginning of 1857, 

 led the author, as Chief Engineer in charge of the Expedition, to 

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

 saving time in ascertaining the levels of the mountain passes of that 

 Island. The aneroids offered by makers did not appear sufficiently 

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

 gestion Messrs. 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 M. Bellville, 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- 



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