318 On the Velocity of the calorific Rays 



When the principal axis of the speculum, by means of a 

 telescopic finder of sufficient magnifying power, is directed 

 to the centre of the sun's disk, it may be considered as pa~ 

 rallel to the plane of the ecliptic ; the sun's parallax being 

 so very small as not sensibly to alter the case. For the 

 same reason the plane of the ecliptic may then be regarded 

 as cutting the image at right angles and centrically. The 

 -diameter of the image, corresponding to this common sec- 

 tion, must therefore lie in the direction of the earth's mo- 

 tion in the orbit at the time. The aberration, accordingly, 

 of the sun's rays must lie in the direction of that diameter 

 of the image which, if produced a little at one extremity, 

 must consequently pass through or mark out the vertices 

 of both crescents, should any such be formed by a diffe- 

 rence of aberration of the rays of light and caloric. 



To find out practically this diameter, or directrix as it 

 may be called, some apparatus would be necessary near the 

 focus of the speculum. It might consist of frame-work, so 

 contrived that two harpsichord wires stretched across one 

 another in the same plane, and revolvible on a fixed centre, 

 might be made to intersect at any angle. Let the centre 

 round which the wires turn be placed in the axis of the 

 speculum, at the focus, and the plane of their revolution 

 at right angles to it. Then, on any day at noon, when the 

 centre of the sun's image is brought to the intersection of 

 the wires, turn one of them* into the plane of the meridian, 

 and then the other till it cuts the first at the same angle 

 which the ecliptic does the meridian, at the time, as found 

 by calculation, or nearly so. Then this second wire will 

 lie in the plane of the ecliptic, and in the direction of the 

 earth's motion at the time, and consequently will be the 

 directrix sought, passing through the vertex of each cres- 

 cent, should any such be formed by a difference of aberra- 

 tion. 



Should the experiment be reserved for the noon of the 

 day of either solstice, selecting such as fall very near to 

 noon at the place of trial, a horizontal line, conceived drawn 

 trirough the centre of the image, would then be the direct 

 trix, and would continue so, without an equatorial motion, 

 for such short time as might suffice for some trials of the 

 crescents. 



The same may be done at any time of the day and year, 

 by first setting the two wires to the same angle which the 

 ecliptic makes with the parallel of the place for the time, 

 and then turning round both till the sun's upper or under 



limb 



