200 Queries and Answers. 



Btormy, the general character of the month of July this year, wherever, in the course of a long 

 journey, I happened to be. I merely mention this casually as a note on the weather. I am. 

 Sir, &c. — W. B. Clarke. East Berfrholt, Oct. 5. J 829. 



The Cause of Mirage. — I offer the following supposition as to the cause 

 of the appearance called the mirage, with great humility, if new : — 1. The 

 great heat of the sun draws up the moisture in vapour from the depths of 

 the earth. 2. The same heat so rarefies the atmosphere, as to make it too 

 light to bear the vapour, and consequently it remains hanging on the sur- 

 face, being still kept in vapour by the great heat reflected by the hot sands. 

 — r. Dec. 1829. 



Blectricity. — Can you, through your excellent work, afford your readers 

 some information on the subject of electricity^ which (I speak from know- 

 ledge) would be highly gratifying to some of the junior branches of the 

 families who take it in. Why are flannels and silks more apt to emit sparks 

 this frosty weather than any other substances ? An explanation from any 

 of your correspondents of these and other electrical appearances of the same 

 nature would oblige. Yours, &c. — M. Jan. 1830. 



Effects of Heat and Cold relatively to Vision. — Sir, It has appeared to me, from various cases 

 mentioned by writers, that cold, while it (by condensing substance) diminishes actual size, and 

 Tciagn\l\e& apparent space; and that heat, while it enlarges actual size, diminishes apparent size or 

 space. Take the following from among many : — 1. As to cold, Captain Parry says, '' We had fre- 

 quent occasion to remark, in our walks, the deception which takes place in estimating the distance 

 and size of objects, when viewed over an unvaried surface of snow. It was not uncommon for us 

 to direct our steps towards what appeared a large piece of stone, at about a mile's distance, but 

 which we could take in our hand after about a minute's walk ; nor did we find the deception less 

 on account of its frequency." {Northern Voyages.) In this case I think that it was not the uniform 

 surface of the snow which caused the appearance ; for, had it been, I think, custom would have got 

 used to it, and that it would not any longer deceive them : but, I am inclined to think, it proceeded 

 from the condensed state of the air by cold, at least in part. I do not think that the surface 

 bein-T uniform in general would produce so great an effect, without some other helping cause. The 

 reason why the distance, as well as the object, is evident, for the same cause would affect both, 

 apparent space only being magnified ; and that the same, whether full or empty, occupied or un- 

 occupied, only in the first case our senses cannot perceive it. — 2. As to heat : " Every one who 

 has resided in a southern climate, knows how much the purity of the atmosphere tends to dimi- 

 nish apparent distance. I never saw it produce this effect in so extraordinary a degree as at Mexico 

 (the city). The whole valley is surrounded with mountains, most of which are at least 15 miles 

 from the capital ; yet, on looking down any of the principal streets, it appears directly terminated 

 by a mass of rocks." ( Ward's Travels in Mexico.) How is it that the apparent distance was 

 diminished ? I should think that the apparent size of the mountains was also diminished. This 

 appears to me to be implied by speaking of the mountains as looking like a mass of rocks. Query, 

 Is this the effect of the cold causing the atmosphere to act as a concave mirror (by condensation), 

 and heat causing it to act as a convex mirror (by expansion). — Y Dec. 1829. 



Singular Phenomenon. — Sir, I observed a singular effect this morning, 

 when looking at the snow in my garden, from the drawing-room window. 

 The external cold had condensed, in a slight degree, the vapour on the 

 the windows, but not sufficiently to be frozen. On looking through one of 

 the panes, I remarked a beautiful rose-coloured tinge on all the trees and 

 objects that were covered with snow, although the glass itself was perfectly 

 transparent and colourless, and did not exhibit the same appearance in that 

 part on which the vapour from the warmth of the room had not rested. 

 The other panes, although equally obscured by the vapour, did not produce 

 the same effect, the objects seen through them being of their natural colour. 

 In a quarter of an hour later, on looking again through the same pane of 

 glass, I observed a change in the colour, the objects that were covered with 

 snow having assumed a greenish tinge, while the branches of the trees re- 

 tained the same roseate, or now rather red-brown, hue, all the other parts 

 of the window remaining as before. There was no sun, nor any apparent 

 change in the state of the external atmosphere, the day being gloomy and 

 rather foggy. This phenomenon was distinctly seen by others of the family, 

 to whom I mentioned it, therefore could not proceed from any illusion in 

 my own vision. I should much like to have it accounted for, if any of your 

 correspondents can, from this imperfect description of so singular an appear- 

 ance, ascertain its cause. I am, Sir, &c. — L. F. Rue de Chaillot, Paris, 

 Bee. 22. 1829. 



