560 PHILOSOPHICAL TRANSACTIONS. [aNNO 1 795. 



Fig. 3, is a representation of the imbricated or loricated appearance of the scales which cover part 

 of the sclerotic coat of the eye, divested of its niucles. 



Fig. 4, shows that the scaly appearance is weaker in some birds than in others, according to their 

 different modes of life, more so in the turkey than in the buzzard, (see fig. 3) representing likewise 

 one of the recti muscles attached to the scales. 



Fig. 5, the inside view of these scales in the eye of a turkey, the internal coat of the cornea being 

 torn up or separated from the external. 



Fig. 6, the 4 recti muscles in the eye of the sheep, dissected so as to show their fibres inserted into 

 and going to form the outer coat of the cornea. 



Fig. 7, the i recti muscles of the eye of the turkey, which are partly inserted into and running to 

 form part of tlie outer coat of the cornea. 



Fig. 2, one of the recti muscles, dissected in such a manner as to show that a part of it is inserted 

 into, and the rest of the muscle going to form, the outer coat of the cornea. 



XIII. Observations on the Best Methods of producing Artificial Cold. By Mr. 



Richard Walker, p. 270. 



Having already investigated the means of producing artificial cold, and at the 

 conclusion of my last paper, on the congelation of quicksilver, dismissed that 

 part of the subject, the best method of employing those means naturally becomes 

 a desideratum; to that therefore I have lately given my attention, and flatter 

 myself that the following observations may be considered as a u.seful appendix to 

 my former papers. The freezing point of quicksilver being now as determined 

 a point on the scale of a thermometer, viz. — 39°, as the freezing point of water; 

 and as this metal, exhibited in its solid state, affords an interesting as well as cu- 

 rious phenomenon ; I shall apply what I have to say principally to that object. 



Frequent occasions having occurred to me of observing the superiority of 

 snow, in experiments of this kind, to salts, even in their fittest state, that is, 

 fresh crystallized, and reduced to very fine powder, I resolved on adopting a kind 

 of artificial snow. The first method which naturally presented itself, was by 

 condensing steam into hoar-frost; this answered the purpose, as might be ex- 

 pected, exceedingly well; but the difficulty and expence of materials in collecting 

 a sufficient quantity, induced me to relinquish this mode for another, by which 

 I can easily and expeditiously procure ice in the fittest form for experiments of 

 this kind; the inethod I mean, is by first freezing water in a tube, and after- 

 wards grinding it into very fine powder. Thus possessed of the power of mak- 

 ing ice, and afterwards reducing it to a kind of snow, the congelation of quick- 

 silver becomes a very easy and certain process; for by the use of a very simple 

 apparatus, pi. 7, fig. 7, quicksilver may be frozen perfectly solid, in a few mi- 

 nutes, wherever the temperature of the air does not exceed 85°; thus, ] oz. of 

 nitrous acid is to be poured into the tube b of the vessel, observing not to wet 

 the side of the tube above with it; a circular piece of writing paper of a proper 

 size is to be placed over the acid, resting on the shoulder of the tube, and the 



