History of Mechanical Inventions, §c. 161 



remarked in the Vultur aura. It consists of a kind of itching 

 wart, which often covers the whole of the skin of their head 

 and back of the neck, having a reddish appearance, and suppu- 

 rating with a very fetid greenish humour. The bird thus af- 

 flicted, scratches these warts almost constantly, and the more 

 irritated the larger they grow. In every one of these warts 

 I have found fastened, as a common leech to the real skin, 

 a small worm very like some of those which torment certain 

 quadrupeds, particularly, in this country, the common grey 

 squirrel. I never could ascertain if these parasites killed the 

 birds, but I am certain that many die during winter, or through 

 some means to me unknown. These worms are killed by the 

 bird, as 1 have found many of the warts dried, although large, 

 but without any tenant, after a continuance of cold weather. 

 It is not improbable that the continued filth attached to the 

 head of these birds, after being immersed in the decayed flesh 

 of the animals they feed on, occasions their birth. I have 

 observed this to take place generally with the younger car- 

 rion crows, who, from the tenderness of their skin, are pro- 

 bably more liable to these vermin, and the older ones probably 

 clear themselves of them more easily, as their skulls and skins 

 become tougher. Besides these troublesome settlers, the car- 

 rion crows are troubled with lice and tick-flies of a large size, 

 that never leave them unless they are killed, or the bird dies. 

 The unexpected sight of a powerful enemy always makes 

 these birds instantly disgorge a part of the contents of their 

 stomachs. The object of this is supposed to be to disgust 

 the stranger and make him desist from advancing nearer, 

 but in my opinion it is done to lighten the bird of an extra 

 load, with which it is difficult for it to fly off quickly. This 

 is more probable, as immediately after this discharge the bird 

 takes to its wings. i ' 



Art. XXVI.— HISTORY OF MECHANICAL INVENTIONS 

 AND PROCESSES IN THE USEFUL ARTS. 



1. Account of a New Method of Drawing upon Stone* By M* Laurent, 



Painter, Paris. 

 X he ingenious process which we propose at present to describe, is taken 

 from the report laid before the Institute by Messrs Thenard and De 

 Blainville. 



VOL. VI. NO. I. JAN. 1827. I 



