Astronomical and Nautical Collections. 315 



iii. Observations on Mr. Mendoza y Rioss Method of computing the 

 True from the Apparent Lunar Distances. By Thomas 

 Henderson, Esq. 



It is a general opinion among seafaring persons that the method 

 of Mr. Mendoza y Rios, for computing the true lunar distances, by 

 means of his Tables for Navigation and Nautical Astronomy, the 

 second edition of which was published at London, in 1809, is, for 

 nautical purposes, the most easy, concise, and accurate. But the 

 ingenious author not having fully shown how allowance is to be 

 made for the variation of refraction, arising from a change in the 

 pressure and temperature of the atmosphere, indicated by the ba- 

 rometer and thermometer, and for the effect on the moon's paral- 

 lax, occasioned by the spheroidal figure of the Earth, it may be 

 proper to supply this deficiency. 



As explained by the Author, Tables VI., VII., VIII., and IX., 

 showing the corrections of the apparent altitudes of the sun, stars, 

 and moon, and the auxiliary angles for the computation of the true 

 distances, are constructed for that state of the atmosphere, when 

 the barometer is at 29.6 inches, and Fahrenheit's thermometer at 

 50 degrees. For any other weight and temperature, the variation 

 of refraction is to be added to or subtracted from the corrections in 

 Table VI., accordingly as the true refraction is greater or less than 

 the mean ; but to the corrections in Tables VII. and VIII., and the 

 auxiliary angles in Table IX., the variation of refraction is to be 

 applied in a contrary manner ; that is to say, to be added or sub- 

 tracted, according as the true refraction is less or greater than the 

 mean. The variation of refraction for the auxiliary angles in 

 Table IX. is in every case the same as the variation for the mean 

 refraction 66 seconds, which corresponds to the apparent altitude 

 40° 40 s . After these corrections are made, the true distance is 

 computed as usual. 



The Author, in his explanation of Table VI., has shown how the 

 true refraction is to be calculated. A Table, such as the eighth 

 in Mackay's Treatise on the Longitude, would be useful for exhi- 

 biting the changes of refraction at sight. Or the true refraction 



