PR.OCEEDINGS OF THE POLYTECHNIC ASSOCIATION. 8i7 



than the blue and violet in the spectniui of water-vapor, the color 

 of the vapor should be orange ; this also accounts for the red of 

 the rising and setting sun, or the sun when seen near the horizon. 



A Remarkable Spring. 



Near the center of Marion county, in the State of Florida, is a 

 wonderful fountain called Silver Spring. Out of the rift lime- 

 stone flows a stream, forming a Ijranch of the winding Oclawaha, 

 which is so large that a steamboat comes up to its source and 

 turns about on the stream to go down the next day. A corres- 

 pondent of The. Liberal C//7-i.sfian, who has lately visited the 

 place, descril)es it as a literal fountain 500 feet in width and pour- 

 ing out 30,000,000 of gallons per day; enough to supply ten cities 

 like New York. The glory of this spring is its transparency. 

 Floating on its surface you shudder to look over the edge of your 

 boat. You seem suspended in mid-air. The bottom is near sev- 

 enty feet below you, but every shell, however minute, is distinctly 

 visible. Multitudes of large fish glide about, seeming miracu- 

 lo'.;sly upheld. Occasionally a huge alligator peers out, but 

 retreats from the light into the wonderful tangle of water plants 

 which frinofe the great basin. Cliffs of limestone rise from the 

 bottom almost to the surface. At their base huge rifts show 

 where the vast and unvarying flood wells forth. A delicate pearly 

 wdiite tinges every stone, shell, and twig; while in deeper places, 

 or l)efore the rising and declining sun, the most exquisite tints of 

 green and blue continually lurk and shift and fade. This place is 

 the commercial outlet of a rich country, and Northern enterprise 

 could easily make it a delightful winter home for northern inva- 

 lids. 



A Normal Map of the Solar SpECXRuai. 



Prof. Walcott Gibbs, of Harvard University, in a memoir read 

 before the National Academy of Sciences, described his Normal 

 Map of the Solar Spectrum, in which each spectral line is entered 

 ■according to its wave length as first suggested by Billet. The 

 well know^n Chart of Kirchoft', though executed with great care 

 and labor, is not, properly speaking, normal, since it only repre- 

 sents a spectrum formed by four flint glass prisms, the angles of 

 which, it is true, are given, but of which the indices of refraction 

 are not stated. Moreover the prisms were not placed accurately 

 in the positions, of least deviation for each of the spectral lines. 

 Prof. Gibbs obviates these oljjectious by making a standard map 



