Mechanical Science, 459 



For 1° of Fahrenheit's scale, this is equal to 0^4, or .00010023: 

 ipvhioh may be called one ten -thousandth, without the most trifling 

 error in practice. The barometric column may, therefore, be re- 

 duced to the standard temperature of 32° F. by the following sim- 

 ple rule, which will make a table unnecessary. Before the first 

 three figures of the observed height place two cyphers, multiply by the 

 temperature of the mercury — 32°, and subtract the product from the 

 observed height. Example; barometer 30.597, temperature of mer- 

 cury 74°. 



^40 - 32<i = 4^<* .00305 X 42 = .128 and 30.5^7 - .128 =i 

 30.469 the correct height. 



When the temperature of the mercury is lower thah 32°, the 

 temperature is to be subtracted from 32°, and the product, obtained 

 as before, is to be added to the observed height. Thus, let the ba- 

 rometer be as before^ and the temperature 15° : then 32°— 15° = 

 17;. 00305 x 17 = .052, and 30.597 + 052 = 30.649, the correct 

 height. — Jameson's Journal^ 1827, p. 378. 



1 3. Diamond Lenses. — I see by the last number of the Journal 

 of Science and the Arts, that Mr. Varley has made a Diamond 

 Lens, and also a single microscope with such motions as enable 

 the observer to follow an animalcule in a diagonal direction. It is 

 very odd, but this is precisely my plan for a microscope, which I 

 drew up about four years ago ; and as I could not get any opticiati 

 to undertake it, I sent it to the Society of Arts, and recommended 

 them to offer a premium for the best diamond lens, but they returned 

 it. I have had a microscope of this sort (made by W. and S. Jones, 

 Holborn) about a year and a half, and it answers the purpose com- 

 pletely ; as a person not at all used to microscopes may use a lens 

 of ^^5 inch focus and find a small object with it, and bring any part 

 of it into the field of view with the greatest facility, and follow the 

 motions of an animalcule in a diagonal direction. There are some 

 alterations and improvements, which I have since made, that have 

 rendered it a very complete microscope ; a drawing of which I 

 could'^end you, if you think it would be acceptable. 



I am. Sir, yours, &c. 

 Tringham,^ Norfolk, July 9th, 1827. G. Dakin. 



14. Sapphire Lenses for Single Microscopes. — As it may justly 

 be feared that, notwithstanding the incontestable superiority of 

 diamond lenses, the cost and difficulty attendant on their produc- 

 tion will enhance their value beyond the reach of the public, Mr. 

 A. Pritchard, No. 18, Pickett Street, has applied himself with 

 indefatigable perseverance to the formation of Sapphire Lenses. 

 The valuable experiments of Dr. Brewster have determined that 

 the sapphire possesses a stronger refraction than any other sub- 

 stance capable of giving a single image (diamona excepted), 



