360 PHILOSOPHICAL TRANSACTIONS. [aNNO 1729. 



same parallelism, through the whole orbit of the planet; which is a necessary 

 and obvious consequence of the established laws of motion. 



The 4th article contains an account of some observations made to determine 

 the parallax of Venus in the year 17 l6. The method he used for this purpose, 

 was to take the several distances of time between the appulse of the limb of 

 Venus and of Regulus, which star she passed by about that time, to a horary 

 circle very near the meridian, and to another about 6 hours after, which he 

 measured by the pulses of a watch, of which 143 went to 1 first minute of 

 time. He likewise observed the alteration of those distances taken at the same 

 hour several days, one after another, and allowing a proportional alteration for 

 the time between the two observations, he computed what the difference of 

 their right ascension ought to have been in the latter of them, if there were 

 no parallax; then comparing this difference with that observed, he concluded 

 the disagreement to be the parallax of right ascension. This method the author 

 seems to depend on so much, as to think that an equal degree of exactness is 

 hardly to be expected from any other yet practised. But if we consider that 

 the whole parallax of right ascension amounts, by his observations, to no more 

 than 4 pulses of his watch: and that he allows a possibility of an error of 

 nearly one of those pulses in taking each of the transits, it is evident that if 

 such an error be actually committed in each of the observations on which the 

 finding of the parallax depends, and all of them happen to conspire the same 

 way, the result of all together may possibly be greater than the whole parallax 

 found. On the whole, he makes the horizontal parallax of Venus at that time 

 to have been 24* 20"', and that of the sun 14" 18'"; but as he takes no notice 

 of the latitude of the place, in deducing the horizontal parallax from that of 

 right ascension, they both ought to be increased on that account by about x, or 

 in proportion of 3 to 4. If therefore there be no other mistake in his numbers, 

 the horizontal parallax of the sun, as deduced from his observations, should be 

 about ig*. 



For a telescope of 100 Roman palms he allows an aperture of 3 or 4 inches 

 of that palm, with an eye-glass whose focal length may be from 7 to 1 1 of the 

 same; but what he directs in longer instruments, to increase the breadth of 

 the aperture and focal length of the eye-glass in the same proportion with the 

 instrument, must certainly be the effect of some mistake; for in this case, a 

 longer telescope will magnify no more than the shorter, but only have the 

 strength of light in the object increased in proportion to the square of the 

 length. 



