over, it may still further be reasoned by analogy, that, as the quantity of water transmitted will depend on the capacity of the dike, and the final protection it gives in conveying the fluid, on the length to which it is continued, so, on the other hand, the protection afforded by a lightning rod will also depend on its capacity, and the distance to which it runs. If, in both cases, the length be extended until the force in action be satisfied, the protection received will be as the capacity for transmitting the current: if both be perfect, the protection will be complete; if the dike be not present, the water must be supposed to run loose and undirected; or, if its continuity be frequently interrupted, or narrowed to a small compass, the damage must then be supposed to happen in the intermediate spaces. Such is, in fact, the way in which all bodies of the conducting class already mentioned, (4,) operate in conveying electrical discharges; and it must never be forgotten as an important feature in this discussion, that, whenever we erect an artificial elevation on the earth's surface in the ordinary way, we do in fact, set up a conductor of electricity, upon which the electricity of the atmosphere will fall, and no human power can prevent it. Hence, if metallic bodies be present, those will be first assailed; if not, then the electric matter will fall on the bodies next in conducting power, and so on.

22. A curious illustration of this principle, will be found in an extract from the Memoirs of the Count de Forbin, which is given in the 48th vol. of the Philosophical Transactions. In the night," says the author of these memoirs, it became extremely dark, and it thundered and lightened dreadfully. As we were threatened with the ship being torn to pieces, I ordered the sails to be taken in. We saw upon different parts of the ship above thirty St. Elmo's fires; amongst the rest there was one upon the top of the vane of the mainmast more than a foot and a half in height; I ordered one of the sailors to take it down. When this man was on the top, he heard this fire; its noise resembled that of fired wet gunpowder. I ordered him to lower the vane and come down, but scarcely had he taken the vane from its place, when the fire fixed itself upon the top of the mainmast, from which it was impossible to remove it."

23. Since, then, the conducting power of bodies differs only in degree, and that the action by which they are assailed, is the result of a great natural agent quite independent of them, we may expect to find all bodies liable to be assailed by lightning, though the effects may be most apparent when the conducting power is imperfect. Thus we find cases on record, of ships struck by lightning, in which no metallic spindles were present, or other iron work about the mast head; moreover, it is by no means an uncommon circumstance to find trees and rocks rent asunder by lightning, and to hear of men and quadrupeds, even in a plain and open country, destroyed at the

• See Philosophical Transactions, vols. xlix. and lxix., damage done to the sheer hulk at Plymouth, and on board the Atlas, East Indiaman.

time of a thunder storm, when the electric matter strikes the earth's surface.

[Rep. Pat. Inv.

The land surveyor has his sliding rule; why should not the navigator also have his instrumental calculator? Mr. Crow, late of

Gravesend, now of London, has furnished an admirable answer to this question, in the octant and traverse worker, shown on the last page. The construction of the instrument is so simple, and its utility so very obvious, that we wonder something of the kind has not been invented before. Seamen work their reckoning at present with the help of printed tables of sines, tangents, &c., which are calculated on the principle, that the sine, tangent, and secant of every angle correspond with the sine, tangent, and secant of the circle which measures that angle, and that the sides of the angle are in the same proportion, one to another, as the sines of the opposite angles. Mr. Crow's octant does this work for them mechanically, and with an accuracy to which no pen and ink, or printed calculations, can possibly pretend. The instrument is of a triangular form, each of the three sides representing the sine, tangent, and secant of the opposite angle. The arc is graduated to 45 degrees, which are subdivided by a vernier into minutes; in the ordinary traverse tables, degrees only are given. The following additional explanations we extract from a small descriptive treatise by Mr. Crow, which accompanies the octant:

"There is a double row of figures to the degrees, to denote angles and their complements in either case; that is, when the angle is less than 45 degrees, the lower row become the complement of the given angle, and the contrary-only that the minutes found on the vernier are to be subtracted from 20', and the difference added to the complement will be the degrees and minutes greater than forty-five degrees. When the given angle is less than forty-five degrees, the index is to be placed over it on the arch, in the usual way. Observe always to count from left to right when the angle is less than fortyfive degrees, and from right to left when it is more than forty-five degrees. The same observation applies to the scale of rhumbs or points, which are immediately below; for if the course be three points, it shows at once its complement, five points underneath; and if five points be the given course, then the difference of latitude is found on the slide, and departure on the divided limb. Now for the three sides of the triangle

"First. The diff. of lat. is on that side upon which the slide traverses; and it is also called the divided limb. Here the diff. of lat. is always to be found or laid off, if the course is not more than four points, or forty-five degrees.

"Second. The dep. is the slide, and traverses up and down; upon which the dep. is always to be found or set off, when the course is not more than four points, or forty-five degrees.

"Third. The distance or index. On this side of the triangle the dist. is to be found in the angle of intersection with the slide.

"With respect to the diff. of lat. and dep. one general rule must invariably be observed-it is this: That when the given course is more than four points or forty-five degrees, these sides change names; and so they always do when the diff. of lat. is less than the dep. For if it be a five point course, the diff. of lat. is then to be found on the VOL. X.-No. 5.-NOVEMBER, 1832. 45

slide, and dep. on the limb; and the same is to be observed when the diff. of lat. is less than the dep.; that is, set off the diff. of lat. on the slide, and the dep. on the limb, when these are given to find course and dist.; and the angle of the course thus produced will be found on the arch, and must be read off from right to left, as it will be more than forty-five degrees.*

"Fourth. To find or set off any required number on the sides of the triangle on the common scale, No. 10 to 100 on the limb and index, and to 75 on the slide. Required that the slide should be placed to 53.2 on the diff. of lat. or limb. First, find 55; then take the first subdivision next to this last, is; to this place the edge of the slide, and you will then have on the diff. of lat. side 53.2, the number required. But this number may be called 53.20, thus bringing out the result to two decimal figures, which is equal in accuracy to the table of logarithms, only with this great advantage, that the results are had by the instrument tenfold easier and quicker. The same numbers may also be counted 532. Now the large scale is numbered 1 to 5 on the two longest sides, and 1 to 3 on the slide. This scale is convenient when working for the meridianal diff. of lat. as it frequently happens to run up to thousands. If short boards are made, as two to six miles, which often happens in light airs and calms, any distance is readily set off, and the corresponding results expeditiously obtained. Now to suit our convenience, we may assume the first 10 on either of the sides, on the common scale, as one or unity, and 20 above, as 2, 30 as 3, &c.; then will the first division, which is cut up to the second line, be, and the longer division, &c. &c. In like manner may the first 1 on the large scale be assumed as 10, 2 as 20, S as 30, &c.; then will all the longer divisions from 1 to 2, be each 1, or unity, and the subdivisions between these will each become Also 10. may the 1 on the large scale stand for 100, 2 for 200, &c.; the subdivisions then are 1 each. Now admit the distance run to be six miles on a S. W. by S. or three-point course-place the chamfered edge of the index to 3 on the arch, then bring the slide to cut 60, which now stands for six miles, the dist. run, and it is done. On the limb is 5' for diff. of lat., and on the slide is S'.3, or a little more, for the dep."

1

The treatise from which we have made the preceding extract, contains also numerous examples of the mode of calculating by means of the instrument; and these are so distinct and clear, that he must be a dull sailor indeed, who after reading and working, them, should find himself at a loss to resolve, by means of the octant, any case whatever in practical navigation.

We should think it a matter of regret, were this instrument to supersede altogether what Jack calls head-work; but, we apprehend, it is quite as likely to lead him to the sufficient reason, in most cases, as the means he at present employs. Of books, there is a mechani

• To work and read off by this instrument, it is intended that the heel, or angular point, should be towards the body.

cal use, as well as there is of instruments. To what Mr. Crow says on this head, every inquisitive landsman who has traversed the deep sea, can bear witness.

"How few among that useful class of men, plain practical sailors, are able to go to any extent into the basis of the science of navigation, and who continually are working their various problems without ever thinking further than that they are to add together certain numbers, and subtract others; and that if they do this right, they at last arrive at the desired result! This instrument will insensibly impress upon the minds of those who use it, that in every triangle all the parts bear a rigid and definite relation to each other, and that having certain things given, certain others can thence invariably be found; and that any three things that can be brought to the shape of a triangle, become, thereby, objects of simple computation, whether they are distance, difference of latitude and departure, or the mainmast, a main shroud, and the horizontal line joining that mast and shroud along the deck."

Besides, as the results produced by this instrument must necessarily be perfectly correct, it furnishes an excellent means of verifying computations, and must, on this account alone, be of great use to the young and inexperienced.

We earnestly hope that Mr. Crow will not go without the reward which he deserves for this valuable contribution to nautical practice. He acknowledges to have received liberal support and approval from the Hon. East India Company;" but speaks of the introduction of the instrument into the Royal Navy, as "a hope yet to be indulged." How is this? To whom can the improvement of navigation be of more importance than to the government of this, the first maritime nation in the world? Thousands and tens of thousands have been lavished by it on inventions, (Rotch's fid, for example) of not a tenth part the importance. [Mech. Mag.

¶ Selections from Professor Babbage's Work" on the Economy of Machinery and Manufactures.”

Lace made by Caterpillars.

A most extraordinary species of manufacture, which is, in a slight degree, connected with copying, has been contrived by an officer of engineers, residing at Munich. It consists of lace, and veils, with open patterns in them, made entirely by caterpillars. The following is the mode of proceeding adopted :

Having made a paste of the leaves of the plant, on which the species of caterpillar he employs feeds, he spreads it thinly across a stone, or other flat substance, of the required size. He then with a camel's hair pencil, dipped in olive oil, draws the pattern he wishes the insects to leave open. This stone is then placed in an inclined position, and a considerable number of the caterpillars are placed at