388 PHILOSOPHICAL TRANSACTIONS. [ANNO 1783. 



The caterpillar has 20 feet (6 of its legs being of considerable length, the 

 other 14 very short) and in its first stage is of a jetty black, smooth as to a pri- 

 vation of hair, but covered with innumerable wrinkles. Having acquired its 

 full size, it fixes its hinder parts firmly to the leaf of a turnip, or any other 

 substance, and breaking its outer coat or slough near the head, crawls out, 

 leaving the skin fixed to the leaf, &c. The under coat, which it now appears 

 in, is of a bluish or lead colour, and the caterpillar is evidently diminished in 

 its size. In every respect it is the same animal as before, and continues to feed 

 on the turnips for some days longer : it then entirely leaves off eating, and 

 becomes covered with a dewy moisture, which seems to exude from it in great 

 abundance, and appearing to be of a glutinous nature, retains any loose or pliant 

 substance which happens to come in contact with it, and by this means alone 

 seems to form its chrysalis coat. From the generic characters of the fly Mr. M. 

 concludes it to be a Tenthredo of Hill. 



XIV. On the Shortening of Wire by Lightning. By Mr. Edw. Nairne, 



F.R.S. p. 223. 



In the Philos. Trans, for the year 1780, vol. 70, [Abridg. vol. 14, p. 688,] 

 were printed some experiments of Mr. N.'s, showing the method of shortening 

 of wire by electricity. He afterwards met with a similar circumstance produced 

 by lightning; which was as follows. On the 18th of June, 1782, Mr. Parker's 

 house at Stoke-Newington was struck by lightning, between 2 and 3 o'clock in 

 the afternoon. The lightning passed down the leaden pipe without side the 

 house, which pipe did not reach to the ground by about 10 feet. Here the 

 lightning struck from one of the nails which fastened this leaden pipe to the 

 wall to the end of a crank iron that was driven in the wall opposite to it, within- 

 side the room, and to which was fastened the wire of a night-bolt, rather thicker 

 than usual. This wire was so very loose before the accident happened, that the 

 bolt could not be raised by the handle at the bed-side, so that they were obliged 

 every night to take hold of the bolt itself to lift it up to fasten the door ; but on 

 the night after the accident had happened, on going to bed, they went to raise 

 the bolt up as usual, to secure their chamber-door, when, to their great sur- 

 prize, they found the bolt drawn up ; and on trying to pull it down, they could 

 not with all their strength. Mr. N. went the next day, and not only found the 

 bolt drawn up, but the wire, which before was very loose, and much bent, was 

 drawn very straight, and so tight, that when struck it produced a musical tone. 

 The wire was judged to be shortened several inches ; for, had the wire before the 

 accident been straight, it must have shortened it above 2 inches to have drawn 

 the bolt up. 



The whole length of the wire from the bed-side to the bolt was about 30 



