344 PHILOSOPHICAL TRANSACTIONS. [aNNO 1753. 



These considerations first set Mr. D. on contriving, how to enlarge the field 

 by increasing the number of eye-glasses, without any hindrance to the distinct- 

 ness or brightness of the image : And though others had been about the same 

 work before, yet observing that the five-glass telescopes, sold in the shops, 

 would admit of further improvement, he endeavoured to construct one with the 

 same number of glasses in a better manner; which so far answered expectations, 

 as to be allowed by such persons as are the best judges, to be a considerable im- 

 provement on the former. 



Encouraged by this success, he resolved to try if possible he might gain some 

 further enlargement of the field by the addition of another glass : and by placing 

 and proportioning the glasses in such a manner, as to correct the aberrations as 

 much as possible, without any detriment to the distinctness, he obtained as 

 large a field, as is convenient or necessary, and that even in the longest teles- 

 copes that can be made. 



These telescopes with 6 glasses having been well received, and some of them 

 being gone to foreign parts, it seems a proper time to settle the account of its origin ; 

 which is one of the motives, that has induced him to give this short sketch of the 

 considerations, that gradually led him to its construction. And as the subject 

 has never been fully treated by any author, he intends as soon as may be, to 

 draw up a more particular explanation of the aberrations of light by refraction. 



Xy. j4 Comparison of Different Thermometrical Observations in Siberia. By 

 Mr. Wm. IVatson, F. R. S. p. 108. 



According to the thermometrical observations made by Mons. DemidofF, 

 at Soliskamsky, on the borders of Siberia, latit. 59, in the year 1751, the 

 greatest degree of cold was on Nov. 9, at 7 in the morning, when the thermo- 

 meter, according to Fahrenheit's scale, stood at 34 degrees below O ; which is 

 6Q under the freezing point. This degree of cold, though much greater than 

 what is ever observed in these parts, is little, when compared with the accounts 

 given by Professor Gmelin, in the introduction to the Flora Siberica. This 

 gentleman, who was professor of chemistry and natural history at Petersburg, 

 was sent with several other learned men, to inquire into the natural history of 

 Siberia, and was attended by some students, a painter or two, a miner, and 

 other proper attendants. He continued 9 whole years on this expedition", and 

 the observations he made, extraordinary as some of them are with regard to 

 their truth, are scarcely to be doubted. The mercury in his thermometer, gra- 

 duated according to De L'Isle's scale, often sunk in winter, in very southern 

 parts of this country, as near Selinga, in lat. 48, to near 226, which is equal to 

 55i below in Fahrenheit's thermometer, and is 87^ below his freezing point. 



