Inviijlaii of Ohjicls by Hortzciiital Refraclhn. • 147 



One day, oblerving the land elevated, and feeing a fmall veflel at about eight miles dif- 

 tance, 1 from my window direfted my telefcope to her, and thought her a fitter objeft than 

 any other I had feen for the purpofe of explaining the phenomena of thefe refraftions. The 

 telefcope was forty feet above the level of the fea : the boat's mad about thirty-five feet ; 

 fhe being about twenty to thirty tons burthen. The barometer at 29.7 inches, and Fah- 

 renheit's thermometer at 54°. 



The appearance of the veflel, as magnified in the telefcope, was as reprefented in Fig. 3, 

 and from the ma(l-head to the boom was well defined. I pretty diftinttly faw the head 

 and {boulders of the man at the helm ; but the hull of the veflel was contrafled, confufed, 

 and ill defined. The inverted image began to be well defined at the boom {for 1 could not 

 clearly perceive the man at the helm inverted), and from the boom to the horizon of the fea 

 the fails were well defined ; and I could fee a fmall opening above the horizon of the fea in 

 the angle made by the gaff and maft; and had the mall been {hotter by ten feet (to the 

 height of ^j, the whole would have been elevated above the horizon of the fea, and from 

 )' to d an open fpace. This drawing was taken from a fketch I took at the time, and repre- 

 fents the proportion of the inverted to the ereft objeft, as near as I could take it by the 

 eye ; the former being about two thirds of the latter in height, and the fame breadth re- 

 fpeftively ; though at one time, during my obfervation, which I continued for about an hour, 

 I thought the inverted nearly as tall as the ere£t objeft. The day was fine and clear, with 

 a very light air of wind, and I found very little tremor or ofcillatlon in viewing her through 

 the telefcope. 



I have laid down Fig. 4. for the explanation of the above phenomena, in which A reprefents 

 the window I viewed B the veflel from ; HO the curved furface of the fea ; CD parallel to 

 HO the height of the maximum of denfity of the atmofphere ; the lines marked with the 

 fmall letters a a, bb,cc, dd, the pencils of rays under their various refra£lions from the 

 veflel to the eye or objeft-glafs of the telefcope. 



The pencil of rays a a, from a point near the head of the mainfail, is wholly refra£led in 

 a curve convex upwards, 'oeing everywhere above the maximum of denfity ; and the pencil 

 of rays d d, which ilfues from the fame point in the fail, and pafles near the horizon of the 

 fea at x, is convex upwards from the fail to W, where it pafles the line of maximum of 

 denfity, which is the point of inflexion ; there it becomes convex downwards, pafiing 

 near the horizon at x to y, where it is again infiedled, and becomes convex upwards 

 from thence to the eye. The pencil of rays bb, from the end of the boom, pafRng nearly 

 parallel to the horizon, and near the maximum of denfity, fufFers very little deviation from 

 a right line in the firft part ; but in afcending (from the curvature of the fea) will be con- 

 vex upwards to the eye. The pencil of rays c c, from the fame point in the boom, may 

 have the fmall part to c convex upwards : from ^ to z it will be convex downwards, and 

 from c to the eye convex upwards. 



From this inveftigafion it appears that two pencils of rays cannot pafs from the fame 

 point and enter the eye from the law of refra£tion, except one pencil pafs through a 

 medium which the other has not entered; and therefore the maximum of denfity was 

 below the boom, and could not exceed ten feet of height above the furface of the fea at 

 the time thefe obfervations were made. 



Rcfpc<fling the hull of the vefTel being confufed and ill defined in the telefcope, as 

 by Fig. 3, it arifes from the blending of the rays from the diftlvvnt parts of (lie ObjeO 



U 2 ffrai^ed 



