apO PHILOSOPHICAL TRANSACTIONS. [aNNO 1717. 



the diameters of the planets, Mr. Gascoigne tells him how the measuring 

 glasses, which he had been speaking of, might be applied to a quadrant. " If, 

 says he, here (that is in the distinct base) you place the scale that measures — , 

 or if here a hair be set, that it appear perfectly through the glass — , you may 

 use it in a quadrant, for the finding of the altitude of the least star visible by 

 the perspective wherein it is. If the night be so dark, that the hair or the 

 pointers of the scale be not to be seen, I place a candle in a lantern, so as it 

 cast light sufficient into the glass; which I find very helpful when the moon 

 appears not, or it is not otherwise light enough." 



In another letter, dated Christmas-eve l641, where he. describes the wheel 

 work of his micrometer, and shows how he could apply it to the taking of three 

 points; and specifies his observations of the diameters of the sun and moon, 

 and mentions a theory he had contrived of the sun, &c. and what pains he had 

 taken in the anatomy of the eye, he tells Mr. Crabtric, how he had applied 

 his telescopic sights to a sextant. He says, " Mr. Horrox's Theory of the 

 Moon I shall be shortly furnished to try. For I am fitting my sextant for all 

 manner of observations, by two perspicills with threads. And also I am con- 

 sulting my workman about the making of wheels like j3, y, J", i, diagr. 3, to 

 use two glasses like a sector. If I once have my tools in readiness to my desire 

 I shall use them every night. I have fitted my sextant by the help of the cane, 

 two glasses in it and a thread, so as to be a pleasant instrument, could wood 

 and a country joiner or workman please me." 



In another letter, the date of which is worn out, but is, in Mr. Crabtrie's 

 hand, called his 10th letter to him, he says, " I have given order for an iron 

 quadrant of 5 feet, which will give me the lOOOth part of one degree, which 

 shall be furnished like ray first scale; only my workman is so throng* for my 

 father, that I fear it will not be furnished before the eclipse. I have caused a 

 very strong ruler to be exactly made, and intend to fit it with cursors of iron, 

 with glasses in them and a thread, for my sextant." 



To these I could have added many other passages of the like nature: but 

 these may be sufficient, to show that Mr. Gascoigne, as early as 164O, made 

 use of telescopes on quadrants and sexants, as well as in his invention of the 

 micrometer. 



What commendations these contrivances procured him, and what expectations 

 they raised in some of the astronomers of that time, particularly in two of the 

 most acute of that age, Mr. Horrox and Mr. Crabtrie, may be seen in Mr. 

 Crabtrie's letters to Mr. Gascoigne, which are also in my hands. Some pas- 



• A Yorkshire phrase for fully employed. 



