The sub-floor trench beneath Insulae Block One smelled of damp mortar, crushed brick, and hot fiery sulfur.
Giles Croft crouched in the crawlway beneath the ground-floor slab. His grease-lamp was hooked to an iron bracket in the stone wall. He cast long, sharp shadows across the newly laid stonework.
In his right hand he held a small wooden trowel. In his left, he balanced a box-shaped terracotta tile hollowed with four lengthwise air channels.
“Tubuli tiles,” Giles said, pointing the tip of his trowel along the trench wall. “Ten inches wide, four inches deep. When we bed them into the wall plaster, they form a nonstop upright chimney for warm air. The floor heats by heat flow. The walls heat by rising heat.”
Beside him in the narrow channel, Toby Karr set a red clay chimney pipe tile into a bed of lime mortar. He tapped it level with his trowel handle.
“And the smoke, Lord Giles?” Toby asked, wiping a smudge of brick dust from his chin. “Won’t the soot blacken the bed-chambers?”
“There’s no smoke,” Giles said. “We aren’t routing chimney pipe gases from coal fire bowls. We’re drawing clean air across a copper-tube heat exchanger connected to the main underground steam header. Pure hot air, seventy degrees, forced through the sub-floor labyrinths by natural heat floating lift.”
Toby ran his thumb along the mortar joint. “So the heat moves without a fan or a bellows?”
“Stack effect,” Giles explained, scraping off excess mortar. “Hot air has a lower thickness than cold air. As the air heats in the basement plenum, it rises through the upright wall channels. That created a nonstop low-pressure suction that draws fresh mountain air across the heat exchanger. Simple floating lift-driven breath flow.”
Duncan Stone-Splitter crawled in from the opposite access hatch, shoving a wooden box of mortar ahead of him.
“Master Duncan,” Giles said. “How are the channel clearances?”
“Two feet of clear airspace under the flat stone slabs throughout the ground floor,” Duncan reported. “Every pillar is capped with a flat slate pad. You could drive a terrier through the maze and never touch a wall.”
“Good,” Giles said. “Check the seals on the spreading joints. When steam enters the basement, the heat swelling will move these tiles by half an inch. If the mortar binds too tight, the tiles will spall.”
Giles crawled back toward the basement junction vault.
There, Master Borin Bronze-Lung stood surrounded by copper pipes, brass gate valves, and pressure dials.
The junction vault hummed with a low, vibrant rumble that shook through the bedrock.
A six-inch padded copper main entered from the street tunnel. It was wrapped in three layers of woven wool felt and sealed with pitch-soaked canvas. Inside that pipe ran twelve sea-weights of white-hot underground steam, tapped from the fiery fault eighty meters below.
“Main header is holding at one hundred and sixty degrees, Lord Giles,” Borin reported. He wiped steam cooled water from his copper fire gauge. “The backup copper coils in the air plenum are red-hot. When the air damper opens, the heat could roast a joint of mutton.”
“We don’t need roast mutton,” Giles said, checking the draft pressure gauge. “We need a steady twenty-one degrees in the rooms above.”
Giles stepped to the intake housing.
“Toby, open the intake damper three notches,” Giles ordered. “Borin, crack the cooled water drain valve.”
Borin turned the brass handwheel with a grunt.
A soft, sharp air hiss sighed through the chamber.
In the fresh-air intake shaft, cold mountain air rushed across the boiling-hot copper coil bank. Heated instantly to sixty-five degrees, the floating air surged into the sub-floor maze. It spread out beneath the flat stone slab floors of thirty-two apartments.
Up on the ground floor, Captain Kenneth Drake stood with his boots off. He watched the floor flags with arms crossed over his chest.
Ten minutes passed.
Kenneth lowered his bare toes to the grey stone. His weathered face shifted, a slow, disbelieving relaxation softening the hard lines around his eyes.
“The stone,” Kenneth murmured. “It’s warm as a horse’s flank.”
“Radiant floor heating,” Giles said, climbing the basement stairs and dusting lime from his knees.
Giles stood beside Kenneth in the hallway.
“The human body loses heat primarily through heat exchange with cold surrounding surfaces,” Giles said. “If the floor and lower walls are at twenty-one degrees, you feel comfortable even if the room air is slightly cooler. It eliminates cold drafts and saves thirty percent on air-flow losses.”
Lady Millicent Croft appeared in the hall, followed by two clerks carrying wooden bread-trays. She paused, tilting her head as she felt the rising warmth through the soles of her kidskin slippers.
“The charcoal budget for the domestic quarters has just dropped to zero,” Millicent observed, dipping her pen in ink. “Sixteen tons of cordwood preserved per week across these first two blocks alone. At current market rates, that represents an annual working saving of nine hundred and forty gold sovereigns.”
“And zero chimney fires,” Giles added. “Which saves us the cost of rebuilding thirty dwellings every winter.”
Millicent smiled faintly. “A rare instance where your schemes save money rather than eat it.”
Outside the building, the sub-zero blizzard shrieked against the sealed casement windows, rattling the leaded frames. But inside the long hall, the air was still, warm, and dry.
A heavy knock sounded at the main entrance door.
Kenneth drew his pacing rod, sliding the oak bolt back. The heavy door swung inward, admitting a swirl of fine snow and three tall figures wrapped in desert wool and hardened boiled-leather armor.
At their head stood Chieftain Torvald Dune-Strider.
The desert warlord was tall, sinewy as ash-wood, his dark face weathered by decades of scouring sandstorms. His black beard was braided with silver rings. A broad curved scimitar rested in a shagreen sword sheath at his hip. Behind him stood two clan elders. They shivered beneath their heavy woolen burnouses, their dark eyes wide with worry.
“Lord Croft,” Torvald greeted, placing his right fist over his heart with a crisp click of horn rings. “The cold in this northern pit bites through three layers of camel wool. My elders can barely close their fingers to hold a bread-knife.”
“Come inside, Chieftain Torvald,” Giles said, motioning them past the threshold. “Take off your boots.”
Torvald frowned. “Take off our boots? In the depth of winter?”
“In my apartments, we don’t track grit across fresh volcanic cement screed,” Giles replied deadpan. “Take them off.”
The two clan elders exchanged uneasy glances, but Torvald unbuckled his riding boots. They set them beside the boot-scraper. He stepped onto the grey concrete flags in his coarse wool stockings.
Torvald froze.
His dark eyes snapped wide. He looked down at the floor, then looked up at Giles, his hand dropping from his sword handle.
“What sorcery is this?” Torvald whispered.
He dropped down, pressing both bare palms against the flags.
The stone beneath his hands was pleasantly, wonderfully warm, streaming gentle heat through his calloused skin.
“The earth,” Torvald breathed, turning his face toward his trembling elders. “It’s not frozen rock. It’s the summer sands of the Great Erg. It feels like noon on the red dunes.”
One of the elderly clan leaders sat down beside him, tears welling in his aged eyes as he pressed his wrinkled cheek directly against the warm stone floor.
“A hearth without smoke,” the elder cried softly. “A fire that burns without wood. The ancestors have brought us to a place of peace.”
“It’s not a miracle,” Giles said, rubbing his temple. “It’s a counter-flow hydronic air-exchange hypocaust. Waste heat from the earth-heat fault is circulated through sub-floor tubuli tiles. It’s basic district heat-powered routing.”
Torvald stood up, drawing himself to his full height.
He gripped Giles’s right forearm with an iron grip that smelled of cedar oil and dried leather.
“Lord Master builder,” Torvald declared, his deep voice resounding down the warm hall. “My people were driven from the south by royal tax-butchers. We wandered in the frost until our camels died and our children cried from the cold. You have given my clan thirty warm chambers where our babes won’t freeze in their sleep.”
Torvald drew his curved scimitar three inches from its sword sheath. That let the steel click home with a sharp, ringing ring.
“Two hundred Dune-Strider skirmishers ride under my banner,” Torvald said, eyes burning with fierce loyalty. “From this day forth, their bows and their blades belong to the walls of Grey Crag. Any foe who marches to break your stones will feel the bite of desert venom.”
Giles adjusted his glasses. “Two hundred skirmishers will need twenty-five extra latrine pits and four cords of cabbage a week. I’ll have Lady Millicent draw up the supply boss rations. Kenneth, show the chieftain to Block Three.”
The golden system runes faded, replaced by the quiet, comfortable hum of the air plenums warming the residential rooms above.
“That’s two thousand four hundred refugees off the shanty rows and into heated blocks,” Giles noted, checking off the items in his ledger. “Cordwood consumption across the settlement drops from sixty cords a day to five. That frees twelve forestry teams to clear timber along the northern defensive outer edge.”
“A fine bit of work, my lord,” Kenneth said. He watched the desert clan settle into their quarters. “The men are resting easy. For the first time since the first snow, nobody in camp is coughing up red thick spit.”
Before Giles could reply, a horn blast sounded from the North Barbican.
It wasn’t the steady call of the watch change. It was three wild, jagged blasts—the scout alarm.
Kenneth’s hand flew to his sword. “The northern outer edge.”
Giles followed him through the heavy doorway and out into the biting wind.
Down at the North Gate, a heavy cargo sledge came skidding through the snow, pulled by two exhausted draft horses whose coats were lathered with frosted sweat.
A pathfinder leaped from the sledge rail, waving a torn crimson pennant.
“Scouts down,” the pathfinder shouted across the courtyard. “Ambush at the high moraine. Troll claws. Bring the stretchers.”
Giles ran toward the gate, his boots crunching in the fresh gravel.
In the back of the sledge lay three frontier trappers. Their thick fur parkas were shredded into bloody ribbons. Across the chest of the lead scout, four parallel gouges had torn through boiled leather and flesh, the wound edges blackened not by black flesh-rot, but by jagged needles of blue frost that crackled against the bone.