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LIV. On Transit Instruments, Bij Ez. Walker, Esq. 

 To Mr. Tilloch. 



Astronomers have, in all ages, been very attentive to the 

 construction of instruments for determining the exact time 

 when the sun, or any other celestial luminary, passes over 

 the meridian. The best instrument that has yet been in- 

 vented for this purpose, is the transit telescope; but the 

 difficulty of procuring a proper foundation to fix it upon, 

 and the expense of fitting it up, must render its use ex- 

 tremely limited. Clocks and watches may be regulated for 

 occasional observations, by means of equal altitudes of the 

 sun or stars, taken either with an astronomical quadrant 

 or a Hadley's sextant : but these methods are very incon-- 

 venient for keeping the daily rate of a time-keeper, in con- 

 sequence of the length of the calculations, and the time that 

 is required for taking the observations. Hence it may be 

 presumed, that an instrument for finding the exact time of 

 noon in an easy manner, is still wanted, to supply the place 

 of the transit telescope. 



The method used in former ages for resolving this pro-, 

 blem has, perhaps, been too much neglected since the 

 time that the science of optics began to receive so many 

 improvements. The method to which I allude is that 

 of finding a meridian line by means of the sun's rays 

 transmitted through a small circular aperture made in a 

 piece of metal. In this manner, a kind of transit instru- 

 ment may be constructed, at a small expense, in many situa- 

 tions that afford no foundation for a transit telescope, and 

 to a greater degree of aQcuracy than may, perhaps, be gene- 

 rally supposed. 



Having, some years ago, drawn a line by this method, 

 and being desirous to know how far it deviated from the 

 true meridian, I took 14 equal altitudes of the sun from the 

 surface of a fluid with one of Mr. Stancliffe's best 12-inch 

 sextants. The time of noon, derived from the mean of 



Vol. 24. No. 90. May 180G. T tho«e 



