Up to 1877 the largest pipe used on the oil lines was 4-inch, with the usual steam thread, but the joints leaked under the pressure, 1,200 pounds to the square inch being the maximum the 8-thread pipe would stand. This trouble has been remedied by the 9-thread, taper-cut pipe of the present day, which is tested at the mill to 1,500 pounds pressure, while the average duty required is 1,200 pounds; as the iron used in the manufacture of this line-pipe will average a tensile test strain of 55,000 pounds per square inch, the safety factor is thus about one-sixth. [Illustration: PROFILE SHOWING NATIONAL TRANSIT CO.'S PIPE-LINE, FROM OLEAN TO SADDLE RIVER.] The line-pipe is laid between the stations in the ordinary manner, excepting that great care is exercised in perfecting the joints. No expansion joints or other special appliances of like nature are used on the line as far as we can learn; the variations in temperature being compensated for, in exposed locations, by laying the pipe in long horizontal curves. The usual depth below the surface is about 3 feet, though in some portions of the route the pipe lies for miles exposed directly upon the surface. As the oil pumped is crude oil, and this as it comes from the wells carries with it a considerable proportion of brine, freezing in the pipes is not to be apprehended. The oil, however, does thicken in very cold weather, and the temperature has a considerable influence on the delivery. A very ingenious patented device is used for cleaning out the pipes, and by it the delivery is said to have been increased in certain localities 50 per cent. This is a stem about 21/2 feet long, having at its front end a diaphragm made of wings which can fold on each other, and thus enable it to pass an obstruction it cannot remove; this machine carries a set of steel scrapers, somewhat like those used in cleaning boilers. The device is put into the pipe, and propelled by the pressure transmitted from the pumps from one station to another; relays of men follow the scraper by the noise it makes as it goes through the pipe, one party taking up the pursuit as the other is exhausted. They must never let it get out of their hearing, for if it stops unnoticed, its location can only again be established by cutting the pipe.