290 On Transit Imlrumcnis. 



those observations, diffcrctl only I'-Jl" from the time that 

 the sun passed over the nicridiaii line on the same day. 



But in order to observe the sun's transit over the meridian 

 with greater preci.-^ion, I drew two other lines on each side 

 of the meridian line, which were parallel to it, and at such 

 distances, that the interval of time taken by one of the limbs 

 of the sun's image was about 40 seconds in passing from 

 one line to another, when the sun was near the equator. 

 Those who arc at all acquainted with the use of the transit 

 telescope, will be at no loss to comprehend the use of these 

 lines ; and those who may want directions for the use of 

 that instrument, may consult a treatise on the method of 

 finding the longitude at sea by time-keepers, written by the 

 )ate Mr. William Wales. To those directions I shall only 

 aidd, that the room in which the lines arc drawn must not 

 be ittade too dark during the time of observing, as it is ne- 

 cessary to see the lines before they are illuminated by the 

 sun's image. 



When the solar rays, that flow through a small circular 

 aperture into a room, are received upon the floor, an ellip- 

 tical image will be formed, consisting of a bright central 

 image, and a penumbra svirrounding it. This penumbra 

 renders the contact of the sun's image with a line less cer- 

 tain : but, notwithstanding this inconvenience, I am con- 

 vinced that an experienced obser\'er, under favourable cir- 

 cumstances, will generally determine the time of the sun's^ 

 passage over the meridian line to one-fifth of a second, or 

 less, when the distance between the aperture and the sun's 

 image on the floor measures about 12 feet. Further parti- 

 culars respecting the construction and use of instruments of 

 this description will be submitted to your inspection at some 

 future opportunity. 



I am, sir. 



Your most humble servant, 



Ez. Walker. 



Lynn, May 8, 1806. 



LV. Account 



