12 



served, and warm blankets were then applied to both the western 

 braces, and a deviation to the west, though not above one third of 

 that arising from their application to the alternate braces was the 

 consequence, indicating the difference, not the sum, of the actions 

 in the two cases ; and in this experiment it is very unlikely that the 

 two braces should have been equally heated. 



The next experiment was varied by observing the passage over the 

 1st and 2nd wires, then warming the upper western brace, and ob- 

 serving it on the 3rd, 4th, and 5th wires, allowing the instrument 

 to cool, then again warming the lower western brace to a yet greater 

 degree, and observing on the 6th and 7th wires. The result was a 

 deviation to the east, caused by the first warming, a return to its mean 

 state, and then a deviation to the west from the second warming. 



The experiments were varied by holding the braces some time in 

 the hand, and with similar results. 



From these details, the author concludes that the partial heating 

 of the diagonal braces, or of any one of them, deranges the Cam- 

 bridge transit instrument, according to the reasoning in his former 

 paper ; and that this cause may, in certain instruments, and under 

 certain circumstances of temperature, produce balancing effects, 

 thus giving an appearance of inflexibility which, under other cir- 

 cumstances, would not subsist. 



He then enters into a consideration of the circumstances of Mr. 

 South's experiments, in which, as the braces on the same side were 

 equally heated, the difference of temperature in the upper or lower 

 parts of the tube alone could have operated, and might produce an 

 insensible effect. He combats the idea that the apparent rigidity of 

 Mr. South's instrument arose from its peculiar construction ; as the 

 Greenwich transit, which is similar, and by the same artists, has been 

 found by the assistants, on holding the alternate braces in their hands, 

 to undergo a considerable deviation by the test of the meridian mark. 

 But to be more sure of the nature of the result produced, he requested 

 the Astronomer Royal to try with the Greenwich transit the first 

 experiment mentioned in this paper. He did so, and the results were 

 found to be in accordance with the views entertained by Mr. Wood- 

 house, and are here stated. He concludes this subject by inviting 

 Mr. South to an experiment, decisive, as he conceives, of the ques- 

 tion as far as concerns his instrument, viz. to observe the passage of 

 Polaris in October with one brace exposed to the sun. 



On the subject of the sun's transits, he declares himself unwilling 

 and unprepared to enter, and states himself to have found a difference 

 between the clock's errors, as determined by the sun and the stars, 

 similar to that concluded by Mr. South, and nearly the same in 

 quantity. This, he says, may be partly explained by the increase of 

 the right ascensions of the stars by three tenths of a second, in late 

 catalogues by Mr. Pond, while no corresponding change was made 

 in the catalogue from which the solar tables were computed. 



He considers, on the whole, that the differences in question arise 



