And, still further, they destroy tranquillity by putting us at the mercy of externals. Whatsoever we make necessary for our contentment, we make lord of our happiness. By our eager desires we give perishable things supreme power over us, and so intertwine our being with theirs, that the blow which destroys them lets out our life-blood. And, therefore, we are ever disturbed by apprehensions and shaken by fears. We tie ourselves to these outward possessions, as Alpine travellers to their guides, and so, when they slip on the icy slopes, their fall is our death. If we were not eager to stand on the giddy top of fortune's rolling wheel, we should not heed its idle whirl; but we let our foolish hearts set our feet there, and thenceforward every lurch of the glittering instability threatens to lame or kill us. He who desires fleeting joys is sure to be restless always, and to be disappointed at the last. For, even at the best, the heart which depends for peace on the continuance of things subjected to a thousand accidents, can only know quietness by forcibly closing its eyes against the inevitable; and, even at the best, such a course must end on the whole in failure. Disappointment is the law for all earthly desires; for appetite increases with indulgence, and as it increases, satisfaction decreases. The food remains the same, but its power to appease hunger diminishes. Possession bring indifference. The dose that lulls into delicious dreams to-day must be doubled to-morrow, if it is to do anything; and there is soon an end of that. Each of your earthly joys fills but a part of your being, and all the other ravenous longings either come shrieking at the gate of the soul's palace, like a mob yelling for bread, or are starved into silence; but either way there is disquiet. And then, if a man has fixed his happiness on anything lower than the stars, less stable than the heavens, less sufficient than God, there does come, sooner or later, a time when it passes from him, or he from it. Do not venture the rich freightage of your happiness in crazy vessels. If you do, be sure that, somewhere or other, before your life is ended, the poor frail craft will strike on some black rock rising sheer from the depths, and will grind itself to chips there. If your life twines round any prop but God your strength, be sure that, some time or other, the stay to which its tendrils cling will be plucked up, and the poor vine will be lacerated, its clusters crushed, and its sap will bleed out of it.