LEVELIKG, AND USE OF BAROMETER. 6\) 



refractive power the rays become bent more and more as they ap- 

 proach the earth, and the curve as a rule will be concave towards 

 the earth. 



It would appear an easy matter to determine this curve, but in 

 attempting to do so we at once find a complication and uncertainty 

 arising from accidental causes; a variation of temperature and of 

 the amount of moisture held in suspension, will both separately 

 and combined, cause a variation in the density of the atmosphere, 

 and especially affect Barometric Leveling and often render the 

 results uncertain aui unreliable. 



According to " Bessel " the atmospheric refraction amounts to 

 nearly thirty-five minutes of arc at the horizon and diminishes 

 towards and becomes nil at the zenith. 



Cases may occur where the refraction is negative, i. e., when the 

 curve is convex towards the earth. This happens when the higher 

 strata are condensed, while the lower recieve the heat previously ab- 

 sorbed by the earth or ocean. I have had the opportunity of ob- 

 serving some remarkable cases of negative refraction. In the one 

 case the exhaust of the high-pressure engine of a tug-boat, distant 

 about two miles, stood up distinctly like a row of columns from 

 forty to sixty feet high apparently, at the same time the scraggy 

 cedars four or five miles off appeared like great poplars. In another 

 case the full moon, rising from the horizon of the ocean, presented 

 a remarkable phenomenon; it came up depressed on its upper edge, 

 and continued to rise with its sides perpendicular until the whole 

 disc should have been above the horizon, then it began to assume 

 the form of a balloon until the disc should have been about one- 

 fourth its diameter above the horizon at wiiich time the lower edge 

 left the water, and the disc assumed its usual circular form. This 

 occurred in August, 1867, after a very hot day, the atmosphere, from 

 some cause or another was very much reduced in temperature in the 

 evening, while at the same time the sea was giving out the heat ab- 

 sorbed during the day. 



A singular case occurred to one of the assistants of the Lake 

 Survey. While engaged in taking soundings with two boats within 

 speaking distance, his second boat, seen and spoken but shortly be- 

 fore, suddenly disappeared; upon arising, the boat was plainly visi- 

 ble, whilst sitting it could not be seen. I will not attempt to ex- 

 plain this phenomenon, but I have it from a gentleman of veracity 



