23* Analyses of Books. [Sept. 



in this position it is dipped for a few minutes into hot nitric 

 acid. The silver is melted off, except at the extremities, and the 

 platinum wire remains. The extreme hooks, retaining their 

 silver and size, serve to make the platinum wire visible and 

 tangible. By this process he obtained wires only -i- 8 o 6 tn °f an 

 inch in diameter. 



XVI. Description of a single lens Micrometer. By William 

 Hyde Wollaston, M.D. Sec. R. S.] This instrument possesses 

 the characteristic simplicity and precision which mark all Dr. 

 Wollaston's inventions. It resembles a common telescope with 

 three sliding tubes. Instead of the object-glass is placed a scale 

 of wires, each -^ th of an inch in diameter, and easily counted 

 by means of a regular variation in their length. Instead of the 

 eye-glass is a small single lens, with a focus of about T Vth of an 

 inch. A small perforation is made by the side of this lens within 

 -J^th of an inch of its centre. The substance to be measured is 

 inclosed between two flat glasses, which slide in before the lens, 

 sufficiently near to enable the object to be seen distinctly. The 

 wire, or other substance, to be measured, is seen through the 

 lens, and the scale through the perforation, by the naked eye, 

 and it is seen how many divisions of the scale it covers. In Dr. 

 Wollaston's micrometer one division of the scale corresponds to 

 . , O o 00 th of an inch, when it is at the distance of 166 inches. 

 Hence at the distance of 8 "3 inches it will correspond to v-sW^ 1 

 of an inch. Upon the side of the micrometer are marked the 

 value of one division, according to the distance. 



XVII. Observations of the Winter Solstice of 1812, with the 

 Mural Circle of Greenwich. By John Pond, Esq. Astronomer 

 Royal, F.R.S.] From the observations on the summer solstice, 

 corrected by subsequent observations, Mr. Pond deduced the 

 obliquity of the ecliptic 23° 2/' 5 1 '50": from the winter solstice 

 he deduced it 23° 27' 4.7*35". He thinks it not unlikely that 

 this small discordance may be owing to some slight error in 

 Bradley's refractions ; and he is now employed in endeavouring 

 to ascertain whether Bradley's mean refraction does or does 

 not require alteration. 



XVIII. On the Tusks of the Narwal. By Sir Everard Home, 

 Bart. F. R. S.] Sir Everard had been informed by Mr. Scoresby, 

 jun. that the female narwal had no tusks ; and, in consequence 

 of a scull which he received from that Gentlemen, he wrote the 

 paper on the subject, an account of which appeared in a former 

 Number of the Annals of Philosophy. Subsequent information 

 led him to doubt the accuracy of his information, that the 

 female narwal has no tusks. Mr. Brown having examined all 

 the books on the subject in Sir Joseph Banks's library, found an 

 account of a female narval with two tusks, brought to Ham- 

 burg!!, in 1684, by Dick Peterson, and still to be seen there. 



