On Refraction, 34/ 



before they discovered that very sensible differences were 

 occasioned by these circumstances. 



But all the honour or' introducing corrections on account 

 of the variation of density in the atmosphere, as indicated 

 by the barometer and thermometer, is due to Messrs. Low- 

 thorpe and Hauksbee ; the former of whom, in 16«8, 

 proved by a very simple experiment, in the presence of the 

 Royal Society, that the refractive power of air is directly 

 proportional to its density* : and the layer, by repeating 

 and extending the same course of experiments in the year 

 1708, with the machinery pointed out by the former, found 

 that the variations of refraction, depending on the barome- 

 ter, are proportional to the alteration or height of the mer- 

 cury in the tube : and by a series of these experiments, he 

 furnished us with a table of the corrections which it is ne- 

 cessary to make on account of the changes of heat indicated 

 by the thermometer. These experiments, although not 

 quite conclusive on the subject, were yet made with as 

 much accuracy and care as the nature of the machinery, 

 and the state of experimental philosophy of that time, would 

 admit. An example is also given, towards the end of his 

 paper, on the mode of applying them to correct the refrac- 

 tion. By these, Hauksbee found that a volume of air ex- 

 pressed by unity, when the thermometer was at 130° above 

 zero, became, at 50° below, one-eighth more dense : or, 

 which is the same thing, that the air lost one-eighth of its 

 density, for an elevation of 180 degrees of Fahrenheit's ther- 

 mometer; which is exactly the difference of heat between 

 melting ice and boiling water. But although this one- 

 eighth, as will be shown hereafter, was too small ; yet it 

 laid the foundation for other experiments, since made by 

 several philosophers, by which the quantity of expansion 

 has been determined more accurately. 



We have already hown that the refraction near the pole 

 is greater than in our climate f ; the degree of cold being 

 more intense. It was also found to be less in the torrid zone, 

 where the heat i3 greater than in Europe. Bouguer made a 

 variety of observations at Peru J, the result of which he has 

 given us. In 1740, he came down into an island situaied 

 in the river of Emeralds, called Isle of Inca, where he 

 determined the refraction from 1° to 7° of altitude: and 

 the table which he computed therefrom, shows the refrac- 



* Haukshce's Exper. 4to, 1709, p. 175. 



f It was, however, found by Capt. Phipps, in his voyage to the North, in 

 17 73, that the refraction in latitude SO was the same as in England. But 

 (his was in summer, J Vide Mem. Ac. 1739, and his Fig. de la Terre. 



tion 



