282 Traxsactioxs of the American Ixstitute. 



additions to our knowledge are due on that occasion. By means of 

 tliis wonderful instrument it was found that the protuberances are very 

 unstable and variable in their forms, and in fact merely local aggre- 

 gations of a gaseous medium which entirel}^ surrounds the sun, and 

 may at any time be detected by the spectroscope in any part of the 

 limb. For this envelop, which is a real one, Lockyer proposed the 

 name of " chromosphere," and its structure and chemical composition 

 have since that time been the objects of continual investigation by 

 numerous observers. It has been satisfactorily established that the 

 light which it sends us is chiefly due to incandescent hydrogen, and 

 usually contains indications of the presence of sodium, barium, 

 magnesium, iron, and perhaps other metallic elements. 



Mechanical Origin of tke Sun's Light and Heat. 

 I will exhibit finally two photographs of the eclipse of last August, 

 one by the ISTautical Almanac party, with short exposure, and 

 exhibiting but liftle coronal radiance, and one by the Coast Survey 

 party with much longer exposure, and showing a very considerable 

 glory round the sun. On this occasion I gave my own particular 

 attention to the form of the corona, and the directions of prominent 

 beams, and devoted a large share of the three minutes to measuring 

 its dimensions and sketching its outline. Here are three drawings 

 of its appearance at intervals of a minute, and I think they establish 

 the fiict that for the same observers, and at the same place, its aspect 

 is undergoing continual variation. The facts being now manifest 

 that the forces radiating from the sun cannot be due to combustion, 

 inasmuch as this would be inadequate to afford the supply ; and yet, 

 that tliey must be in process of continual development from sources 

 in which it previously existed in some other forms than as heat and 

 light, since some amount of cooling and fading would otlierwise 

 inevitably be within the range of our detective powers, there remains 

 but one explanation open to us out of all those which science can at 

 present suggest. This is that the light and heat are the results of 

 mechanical action, and that forces which were prcA'iously engaged in 

 producing motion, are by the arrest of that motion, made to appear 

 in this new form, just as iron grows hot under the blows of a hammer, 

 or an axle takes fire in consequence of friction. From this inference 

 there seems indeed no escape ; but what is the motion which is thus 

 converted ? And whore are the moving bodies, which are endowed 

 with such mighty force, and are encountering such enormous resistance 



