36 



SCIENCE. 



[N. S. Vol. II. No. 28. 



able with the normal spectrum. Bolograpliy 

 is the name given by Professor Langley to 

 these processes which, by the joint use of 

 the bolometer and photography, have auto- 

 matically produced a complete chart of the 

 invisible heat spectrum equal in length to 

 ten times the entire luminous spectrum of 

 the Sun, though indications of heat extend 

 still farther. A fuller account of Professor 

 Langley's significant work will be found in 

 Nature, Vol. 51, (1894), p. 12, and in the 

 new Astrophysical Journal for February, 1895, 

 published at the University of Chicago. 

 The bolometer was built by Grunow of N'ew 

 York, and forms part of the equipment of 

 the astrophysical observatory of the Smith- 

 sonian Institution at Washiagton. So sen- 

 sitive is this delicate instrument that it is 

 competent to detect a temperature fluctua- 

 tion as minute as the millionth part of a 

 degree centigrade. It is proper to add that 

 the researches conducted with such an in- 

 strument, often appearing remote and mean- 

 ingless to a layman, are eminently practical 

 in their bearing, as they pertain directlj' to 

 the way in which the Sun affects the Earth, 

 and man in his relations to it; and to the 

 method of distribu^tion of solar heat, form- 

 ing thus, among other things, a scientific 

 basis for meteorology. 



At the end of the solar spectrum remote 

 from the red is the ultra-violet region, or- 

 dinarily invisible ; a portion of whicli may, 

 however, be seen by receiving it upon 

 uranium glass or other fluorescent sub- 

 stances. Glass being nearlj^ opaque to the 

 short waVe-lengths of violet and ultra-violet, 

 the optical parts of instruments for this re- 

 search are made of quartz or calc-spar, 

 or the necessary dispersion is obtained 

 by using the diifraction grating. The 

 superior intensitj^ of the chemical or actinic 

 rays in this region renders photography of 

 especial service ; and sensitive films stained 

 with various dyes have been effectively em- 

 ployed. The painstaking investigations of 



Rutherfurd, Cornu, H. Draper, Eowland 

 and Vogel have provided splendid maps of 

 the invisible ultra-violet spectrum, exceed- 

 ing many times the length of the visible 

 spectrum. The farther region of the ultra- 

 violet is pretty abruptly cut off by the ab- 

 sorptive action of our atmosphere. 



The constant of solar heat, fii'st investi- 

 gated by Herschel and Pouillet in 1837-38, 

 was redetermined by Professor Langley in 

 1881. He adopts three calories (small) as the 

 solar constant, which signifies that " at the 

 Earth's mean distance, in the absence of its 

 absorbing atmosphere, the solar rays would 

 raise one gramme of water three degrees 

 centigrade per minute for each normally ex- 

 posed square centimetre of its surface. * * * 

 Expressed in terms of melting ice, it impUes 

 a solar radiation capable of melting an ice- 

 shell 54.45 metres deep annually over the 

 whole surface of the Earth." Professor 

 Langley's Researches on Solar Seat and its 

 Ahsorptionhy the Ea^ih^ s Atmosphere ; A Report 

 of the Mount Whitney Expedition, were pub- 

 lished as No. sv. of the Professional Papers of 

 the Signal Service (Washington, 1884). 



To express the solar heat in terms of en- 

 ergy: When the Sun is overhead, each 

 square metre of the Earth's surface receives 

 (deducting for atmospheric absorption) an 

 amount of heat equivalent to 1^ horse- 

 power continuously. In solar engines like 

 those of Ericsson and Mouchot about ^ of 

 this is virtually wasted. Of heat radiation 

 emitted from the Sun and passing along its 

 radius. Professor Frost finds that about ^ 

 part is absorbed in the solar atmosphere, 

 which, were it removed, would allow the 

 Eai'th to receive from the Sun 1.7 times the- 

 present amount. Imagine the hemisphere 

 of our globe turned towards the Sun to be 

 covered with horses, ai-ranged as closely to- 

 gether as possible, no horse standing in the 

 shadow of any other; then cover the oppo- 

 site hemisphere with an equal number of 

 horses: the solar energj^ intercepted bj'^ the 



