104 Dr. Andrews on a Method of obtaining a perfect Vacuum 



The red colour in the present instance appeared sometimes 

 more intense than is generally observed on plains. "When the 

 light falls upon the fog, it exhibits the usual uniform gray, 

 similar to a mixture of 91 white with 9 per cent, black. 



A few words now remain to be said upon the duration of the 

 twilight. It is everywhere known in the Alps, that on high 

 mountains the duration is longer, although this is sometimes 

 over-estimated. It may be almost regarded as a tradition re- 

 peated for every mountain, even when it is but a few thousand 

 feet high, that the evening and the morning twilight touch each 

 ■other at midnight. Though this is an exaggeration, a difference 

 in the duration of the twilight is very appreciable in the higher 

 regions. As the horizon expands from an Alpine summit, it is 

 evident that the higher we ascend the greater will be the arch 

 which separates sunrise from sunset, and hence the longer the 

 day. In valleys, on the contrary, it often occurs that the direct 

 sunlight is held back by interposed mountains, and hence is 

 present only a few hours of the day. The feeble twilight is also 

 considerably diminished by the same cause ; thus the position of 

 valleys with regard to the horizon may be such, that night sets in 

 very soon after the setting of the sun*. The twilight, in our lati- 

 tude, continues on an average upon the plains until the sun has 

 descended 1 8° under the horizon. Upon mountains the sun attains 

 a much greater depth before the twilight departs*. It is diffi- 

 cult to express this with exactness, as the alterations in the 

 transparency of the atmosphere on different days exercise so 

 considerable an influence. 



XV. On a Method of obtaining a perfect Vacuum in the Receiver 

 of an Air-pump. By Thomas Andrews, M.D., F.R.S., 

 M.R.LA.f 



THE space left vacant in the upper part of a long glass tube, 

 which after being filled with mercury is inverted in a basin 

 of the same metal, affords the nearest approach to a perfect 

 vacuum which has hitherto been obtained. It is true that it 

 contains a little mercurial vapour at the ordinary temperature 

 of our summers, and probably also at lower temperatures ; but 

 the quantity is exceedingly small, and its influence in depressing 

 the barometric column must be altogether inappreciable. Besides 

 the mercurial vapour, a trace of air may generally be detected 

 even in tubes which have been carefully filled, and in which the 

 air interposed between the glass and mercury has been expelled 



* Compare also Martin's Monit. Univers' 1844, p. 2/.% ; and Ksemtz, 

 Lehrbuch dc MeteoroL, vol. iii. p. 50 and following, 

 t Communicated by the Author. 



