Further examination of the map will show that there are two lines, and only two, along which an invasion of India might be conducted. On the one hand, the mountains become very narrow just north of the head of the Kabul River. There in fact a single though lofty ridge, the Hindu Kush, is all that separates the basin of the Oxus from that of the Indus. As we see from the map, low ground is very near on the two sides of the Hindu Kush. The way into India over the passes of the Hindu Kush is known as the Khyber route, from the name of the last defile by which the track descends into the Indian Plain. If we now look some five hundred miles to the southwest of Kabul, we see that the Afghan mountains come suddenly to an end, and that a pathway leads round their fringe from Herat to the Indus Basin, passing along the border of Seistan. From Herat to beyond Kandahar, this way lies over an upland plain and is easy, but the last part of the journey is through a mountainous district down to the lowland of the Indus. This is the Bolan route, so called from the last gorge towards India. It will be noticed that the Bolan route debouches upon the Indus opposite to the great Indian Desert. Therefore it is that the Khyber route has been the more frequented. It leads directly between the desert and the mountain foot, upon the inner gateway of India at Delhi. We conquered the Punjab from the Sikhs, but for many centuries it had been ruled by the Musulmans. In the break up of the Mogul Empire invaders had come, during the eighteenth century, from Persia and from Afghanistan, who carried devastation even as far as Delhi. Thus it was that with relative ease the Sikhs as contemporaries of the Marathas established a dominion in the helpless Punjab. They extended their rule also into the mountains of Kashmir, north of Lahore. Let us commence our survey of the northwest at Dehra Dun, which is placed in a mountain valley among the foot hills of the Himalayas, not far from the hill station of Mussoorie, of which we heard in the last lecture. Then from Dehra Dun we will travel two hundred miles northwestward, crossing the Beas, one of the five rivers of the Punjab, to Amritsar, the holy city of the Sikhs. Fifty miles west of Amritsar, on the Ravi, another of the Indus tributaries, is Lahore, the traditional capital of the Punjab. From Lahore onward we traverse irrigated strips of fertile ground, with sandy plains intervening, with a scanty herbage for a few camels. Then follows a broken and more desolate country in the north of the Punjab. So we come to the Indus itself, and beyond this, nearly three hundred miles from Lahore, to the military station of Peshawar, the last Indian city on the great track leading northwestward from Calcutta, through Allahabad and Delhi. Not far from Peshawar is the Khyber Pass.