VOL. LXXXIII.] PHILOSOPHICAL TRANSACTIONS. 305 



maximum that this glass will bear ; with 500 the image is not so well defined ; 

 with 200 or 300, it is beautifully distinct and bright ; but this inquiry demands 

 more experiments than I have hitherto made, having been able to procure these 

 high powers only within a few weeks. 



Sir G. then gives ample directions for adjusting the parts of this instrument, 

 and the method of using it. 



XL Additional Observations on the Method oj Making Ice at Benares. By John 

 Lloyd Williams, Esq., of Benares, p. 129. 



In addition to what has already been communicated, respecting the mode of 

 procuring ice in this country, the following observations on that subject, accom- 

 panied with some account of the temperature of the air, and state of the thermo- 

 meter, may not be unacceptable. April 30, 1792, the thermometer, in the 

 shade, being at 95°, some water was taken up from a well, 60 feet deep, and 

 the thermometer being immerged in it, its temperature was found to be 7 A de- 

 grees. This water was then poured into 4 pots, or pans, similar to the former 

 ones. They were also similar to each other in size and construction, except that 

 1 of them were new and unglazed, and the other 1 old, with their pores closed, 

 so that no moisture could transpire through them. These pots were then ex- 

 posed to a hot westerly wind, in the shade, for 3 hours ; viz. from 1 o'clock in 

 the afternoon till 5. On examining them at that time, the water in the old pots 

 was found to be at 84°, and that in the new, or porous ones, at 68. After re- 

 maining in that situation 1 hour longer, the water in the old pots rose to 88'', 

 while that in the new ones continued at 68. 



May Ist, at 1 o'clock in the afternoon, the thermometer then being, in the 

 sun, at 110°, and in the shade at 100°, the experiment was repeated, with the 

 same pots as before. After being filled with well-water, they were exposed for 

 4 hours, viz. from 2 o'clock till 6, to a hot wind; the water in the old pots was 

 then found to be at 97°, that in the new ones at 68. 



The foregoing observations on the frigorific effect of evaporation from porous 

 vessels, will perhaps account, in some measure, for ice being formed when the 

 thermometer, in the air, is above the freezing point. And the power of eva- 

 poration in generating cold, may be further elucidated by the following observa- 

 tions on the effects produced, by its means, in our houses. May 16, 1792, at 

 1 in the afternoon. 



The thermometer, in the sun, with a hot westerly wind, rose to 118° 



Ditto, in the shade, but exposed to the hot wind 110 



Ditto, in the house, which was kept cool by tatties 87 



June 7- Thermometer, in the sun 1 13 



Ditto, in the shade, and hot wind 104 



Ditto, in the house, cooled by tatties 83 



VOL. XVII. R R 



